Fix test_source_file_does_not_override_standard_library to work with Python 3.9.

Overloading the "re" module causes weird errors on our new Windows VM image, probably because Python itself uses it for some purpose now. I noticed this in our Bazel CI testing area, so the new image is not yet live.

Let's try to use a module that's less core to Python, but still in the standard library - I picked "mailbox" after looking at the list of standard library modules.

Here's an example log from the failing test on the new image:

```
** test_source_file_does_not_override_standard_library *************************
[...]
Error processing line 1 of C:\Python3\lib\site-packages\protobuf-3.17.3-py3.9-nspkg.pth:

Fatal Python error: init_import_site: Failed to import the site module
Python runtime state: initialized
Traceback (most recent call last):
  File "C:\Python3\lib\site.py", line 169, in addpackage
    exec(line)
  File "<string>", line 1, in <module>
  File "C:\Python3\lib\importlib\util.py", line 2, in <module>
    from . import abc
  File "C:\Python3\lib\importlib\abc.py", line 17, in <module>
    from typing import Protocol, runtime_checkable
  File "C:\Python3\lib\typing.py", line 2203, in <module>
    Pattern = _alias(stdlib_re.Pattern, 1)
AttributeError: module 're' has no attribute 'Pattern'
```
PiperOrigin-RevId: 389572773

Author: philwo

src/test/shell/bazel/python_version_test.sh

@@ -531,13 +531,13 @@ py_library(
     # (i.e., those implemented in C) use a different loader than
     # Python-implemented ones, even though they're both part of the standard
     # distribution of the interpreter.
-    srcs = ["re.py"],
+    srcs = ["mailbox.py"],
 )
 EOF
   cat > test/main.py << EOF
-import re
+import mailbox
 EOF
-  cat > test/re.py << EOF
+  cat > test/mailbox.py << EOF
 print("I am lib!")
 EOF