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.