test/alternator: un-xfail a test which passes on modern Python

We had an xfailing test that reproduced a case where Alternator tried
to report an error when the request was too long, but the boto library
didn't see this error and threw a "Broken Pipe" error instead. It turns
out that this wasn't a Scylla bug but rather a bug in urllib3, which
overzealously reported a "Broken Pipe" instead of trying to read the
server's response. It turns out this issue was already fixed in
   urllib3/urllib3#1524

and now, on modern installations, the test that used to fail now passes
and reports "XPASS".

So in this patch we remove the "xfail" tag, and skip the test if
running an old version of urllib3.

Fixes scylladb#8195

test/alternator/test_manual_requests.py

@@ -8,7 +8,9 @@
 import pytest
 import requests
 import json
+import urllib3
 from botocore.exceptions import BotoCoreError, ClientError
+from packaging.version import Version
 
 def gen_json(n):
     return '{"":'*n + '{}' + '}'*n
@@ -100,8 +102,10 @@ def test_too_large_request(dynamodb, test_table):
 #    Content-Length header or chunked encoding (reproduces issue #8196).
 # 2. The client should be able to recognize this error as a 413 error, not
 #     some I/O error like broken pipe (reproduces issue #8195).
-@pytest.mark.xfail(reason="issue #8195, #8196")
+@pytest.mark.xfail(reason="issue #8196")
 def test_too_large_request_chunked(dynamodb, test_table):
+    if Version(urllib3.__version__) < Version('1.26'):
+        pytest.skip("urllib3 before 1.26.0 threw broken pipe and did not read response and cause issue #8195. Fixed by pull request urllib3/urllib3#1524")
     # To make a request very large, we just stuff it with a lot of spaces :-)
     spaces = ' ' * (17 * 1024 * 1024)
     req = get_signed_request(dynamodb, 'PutItem',
@@ -114,15 +118,16 @@ def generator(s):
     # request succeeded, and the status_code was 200 instead of 413.
     assert response.status_code == 413
 
-@pytest.mark.xfail(reason="issue #8195")
 def test_too_large_request_content_length(dynamodb, test_table):
+    if Version(urllib3.__version__) < Version('1.26'):
+        pytest.skip("urllib3 before 1.26.0 threw broken pipe and did not read response and cause issue #8195. Fixed by pull request urllib3/urllib3#1524")
     spaces = ' ' * (17 * 1024 * 1024)
     req = get_signed_request(dynamodb, 'PutItem',
         '{"TableName": "' + test_table.name + '", ' + spaces + '"Item": {"p": {"S": "x"}, "c": {"S": "x"}}}')
     response = requests.post(req.url, headers=req.headers, data=req.body, verify=False)
     # In issue #8195, Alternator closed the connection early, causing the
-    # library to throw an exception (Broken Pipe) instead noticing the error
-    # code 413 which the server did send.
+    # library to incorrectly throw an exception (Broken Pipe) instead noticing
+    # the error code 413 which the server did send.
     assert response.status_code == 413
 
 def test_incorrect_json(dynamodb, test_table):