Deprecate Python 3.6 support

[twm]

https://twistedmatrix.com/trac/ticket/10303

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/10303
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• The title of the PR starts with the associated Trac ticket number (without the # character).
• I have updated the automated tests and checked that all checks for the PR are green.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.

[adiroiban]

Thanks for the update.

I think that py3.6 should also be removed from the CI

Also, I think that it would help to do a cpython and pypy 3.6 cleanup in this PR

ex

diff --git a/src/twisted/cred/test/test_cred.py b/src/twisted/cred/test/test_cred.py
index 08e7ed0a1e..d797894ecf 100644
--- a/src/twisted/cred/test/test_cred.py
+++ b/src/twisted/cred/test/test_cred.py
@@ -266,11 +266,6 @@ class HashedPasswordOnDiskDatabaseTests(unittest.TestCase):
 
     def hash(self, u: bytes, p: bytes, s: bytes) -> bytes:
         hashed_password = crypt(p.decode("ascii"), s.decode("ascii"))  # type: ignore[misc]
-        # workaround for pypy3 3.6.9 and above which returns bytes from crypt.crypt()
-        # This is fixed in pypy3 7.3.5.
-        # See L{https://foss.heptapod.net/pypy/pypy/-/issues/3395}
-        if isinstance(hashed_password, bytes):
-            return hashed_password
         return hashed_password.encode("ascii")
 
     def testGoodCredentials(self):
diff --git a/src/twisted/internet/defer.py b/src/twisted/internet/defer.py
index 802dad5ee0..2be87ed300 100644
--- a/src/twisted/internet/defer.py
+++ b/src/twisted/internet/defer.py
@@ -1681,11 +1681,7 @@ def _inlineCallbacks(
             appCodeTrace = traceback.tb_next
             assert appCodeTrace is not None
 
-            if version_info < (3, 7):
-                # The contextvars backport and our no-op shim add an extra frame.
-                appCodeTrace = appCodeTrace.tb_next
-                assert appCodeTrace is not None
-            elif _PYPY:
+            if _PYPY:
                 # PyPy as of 3.7 adds an extra frame.
                 appCodeTrace = appCodeTrace.tb_next
                 assert appCodeTrace is not None
diff --git a/src/twisted/internet/interfaces.py b/src/twisted/internet/interfaces.py
index 9eca93fd56..b638a86e20 100644
--- a/src/twisted/internet/interfaces.py
+++ b/src/twisted/internet/interfaces.py
@@ -1086,8 +1086,8 @@ class IReactorProcess(Interface):
         L{bytes} using the encoding given by L{sys.getfilesystemencoding}, to be
         used with the "narrow" OS APIs.  On Python 3 on Windows, L{bytes}
         arguments will be decoded up to L{unicode} using the encoding given by
-        L{sys.getfilesystemencoding} (C{mbcs} before Python 3.6, C{utf8}
-        thereafter) and given to Windows's native "wide" APIs.
+        L{sys.getfilesystemencoding which is C{utf8}}
+        and given to Windows's native "wide" APIs.
 
         @param processProtocol: An object which will be notified of all events
             related to the created process.
diff --git a/src/twisted/mail/test/test_pop3client.py b/src/twisted/mail/test/test_pop3client.py
index 6842df0806..9dc2ace4b1 100644
--- a/src/twisted/mail/test/test_pop3client.py
+++ b/src/twisted/mail/test/test_pop3client.py
@@ -630,10 +630,6 @@ class POP3ClientModuleStructureTests(TestCase):
         ]
 
         for pc in publicClasses:
-            if sys.version_info < (3, 7) and pc == "List":
-                # typing.List shows up in publicClasses on
-                # Python < 3.7, so skip it.
-                continue
             if not pc == "POP3Client":
                 self.assertTrue(
                     hasattr(twisted.mail.pop3, pc),
diff --git a/src/twisted/test/test_sslverify.py b/src/twisted/test/test_sslverify.py
index faae0ac3c3..b4257fc220 100644
--- a/src/twisted/test/test_sslverify.py
+++ b/src/twisted/test/test_sslverify.py
@@ -3342,12 +3342,8 @@ class SelectVerifyImplementationTests(SynchronousTestCase):
         )
 
         importErrors = [
-            # Python 3.6.3
-            "'import of service_identity halted; None in sys.modules'",
             # Python 3
             "'import of 'service_identity' halted; None in sys.modules'",
-            # Python 2
-            "'No module named service_identity'",
         ]
 
         expectedMessages = []
diff --git a/src/twisted/web/test/test_http.py b/src/twisted/web/test/test_http.py
index 7ffea4e0bc..4b3614e8de 100644
--- a/src/twisted/web/test/test_http.py
+++ b/src/twisted/web/test/test_http.py
@@ -2422,17 +2422,6 @@ ok
 
 
 class QueryArgumentsTests(unittest.TestCase):
-    # FIXME: https://twistedmatrix.com/trac/ticket/10096
-    # Re-enable once the implementation is updated.
-    @skipIf(sys.version_info >= (3, 6, 13), "newer py3.6 parse_qs treat ; differently")
-    def testParseqs(self):
-        self.assertEqual(parse_qs(b"a=b&d=c;+=f"), http.parse_qs(b"a=b&d=c;+=f"))
-        self.assertRaises(ValueError, http.parse_qs, b"blah", strict_parsing=True)
-        self.assertEqual(
-            parse_qs(b"a=&b=c", keep_blank_values=1),
-            http.parse_qs(b"a=&b=c", keep_blank_values=1),
-        )
-        self.assertEqual(parse_qs(b"a=&b=c"), http.parse_qs(b"a=&b=c"))
 
     def test_urlparse(self):
         """

[adiroiban]

and there is https://twistedmatrix.com/trac/ticket/10096 which I think that it can be closed , now that we no longer support 3.6

[adiroiban]

Also, I think that we can remove the contextvars dependency as it's provided by stdlib in 3.7 and newer

[twm]

Hi @adiroiban, this is just to deprecate support. Dropping support is https://twistedmatrix.com/trac/ticket/10304 and it can't be done until a year after the depreciation, as I understand the compatibility policy. So the CI updates and cleanup will have to wait until then.

[adiroiban]

I think the deprecation policy only applies to twisted API.

But yes, you are right.

Let's merge this to make the announcement and later we can look at the cleanup.