bpo-46425: fix direct invocation of test_contextlib

[sobolevn]

There was a problem with direct invocation of test_contextlib. The best way to illustrate it is:

» ./python.exe Lib/test/test_contextlib.py
...................................................................................
----------------------------------------------------------------------
Ran 83 tests in 0.046s

OK

But, when a path starts with ./, we have a problem:

» ./python.exe ./Lib/test/test_contextlib.py
............................FFF....................................................
======================================================================
FAIL: test_exception (__main__.TestChdir)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython/./Lib/test/test_contextlib.py", line 1159, in test_exception
    self.assertEqual(os.getcwd(), target)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: '/Users/sobolev/Desktop/cpython/Lib/test/data' != '/Users/sobolev/Desktop/cpython/./Lib/test/data'
- /Users/sobolev/Desktop/cpython/Lib/test/data
+ /Users/sobolev/Desktop/cpython/./Lib/test/data
?                                ++


======================================================================
FAIL: test_reentrant (__main__.TestChdir)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython/./Lib/test/test_contextlib.py", line 1143, in test_reentrant
    self.assertEqual(os.getcwd(), target1)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: '/Users/sobolev/Desktop/cpython/Lib/test/data' != '/Users/sobolev/Desktop/cpython/./Lib/test/data'
- /Users/sobolev/Desktop/cpython/Lib/test/data
+ /Users/sobolev/Desktop/cpython/./Lib/test/data
?                                ++


======================================================================
FAIL: test_simple (__main__.TestChdir)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython/./Lib/test/test_contextlib.py", line 1132, in test_simple
    self.assertEqual(os.getcwd(), target)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: '/Users/sobolev/Desktop/cpython/Lib/test/data' != '/Users/sobolev/Desktop/cpython/./Lib/test/data'
- /Users/sobolev/Desktop/cpython/Lib/test/data
+ /Users/sobolev/Desktop/cpython/./Lib/test/data
?                                ++


----------------------------------------------------------------------
Ran 83 tests in 0.050s

FAILED (failures=3)

When __file__ is normalized - the error is gone. Now we can use both styles to call this module.

https://bugs.python.org/issue46425

CC @corona10 as my mentor.

[miss-islington]

Thanks @sobolevn for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10.
🐍🍒⛏🤖

[miss-islington]

Sorry, @sobolevn and @serhiy-storchaka, I could not cleanly backport this to 3.10 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 22f73bd9f1fc573d5c998f345b66c29f7ca6614d 3.10

[miss-islington]

Sorry @sobolevn and @serhiy-storchaka, I had trouble checking out the 3.9 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 22f73bd9f1fc573d5c998f345b66c29f7ca6614d 3.9

[sobolevn]

Thank you, @serhiy-storchaka! I will backport them manually.