Don't remove parens in asserts with assigment expr

Black would previously strip the parenthesis away from assert statement like this one:

    assert (spam := 12 + 1)

Which happens to be invalid code. Now before making the parenthesis invisible, Black
checks if the assignment expression's parent is an assert stamtment, aborting if True.

I added a bunch of test cases from the PEP defining asssignment expressions (PEP 572).

CHANGES.md

@@ -8,6 +8,9 @@
   in the name of the cache file. Without this fix, changes in these flags would not take
   effect if the cache had already been populated. (#2131)
 
+- Don't remove necessary parentheses from assignment expression containing assert
+  statements. (#2143)
+
 ### 21.4b0
 
 #### _Black_

src/black/__init__.py

@@ -5613,7 +5613,7 @@ def maybe_make_parens_invisible_in_atom(node: LN, parent: LN) -> bool:
         return False
 
     if is_walrus_assignment(node):
-        if parent.type in [syms.annassign, syms.expr_stmt]:
+        if parent.type in [syms.annassign, syms.expr_stmt, syms.assert_stmt]:
             return False
 
     first = node.children[0]

tests/data/pep_572_remove_parens.py

@@ -17,6 +17,25 @@
 
 a, b = (test := (1, 2))
 
+# see also https://github.com/psf/black/issues/2139
+assert (foo := 42 - 12)
+
+foo(x=(y := f(x)))
+
+
+def foo(answer=(p := 42)):
+    ...
+
+
+def foo2(answer: (p := 42) = 5):
+    ...
+
+
+lambda: (x := 1)
+
+# we don't touch expressions in f-strings but if we do one day, don't break 'em
+f'{(x:=10)}'
+
 # output
 if foo := 0:
     pass
@@ -36,3 +55,22 @@
 test: int = (test2 := 2)
 
 a, b = (test := (1, 2))
+
+# see also https://github.com/psf/black/issues/2139
+assert (foo := 42 - 12)
+
+foo(x=(y := f(x)))
+
+
+def foo(answer=(p := 42)):
+    ...
+
+
+def foo2(answer: (p := 42) = 5):
+    ...
+
+
+lambda: (x := 1)
+
+# we don't touch expressions in f-strings but if we do one day, don't break 'em
+f"{(x:=10)}"