_build_tables: Invalidate cache before importing generated modules

[mgorny]

Make sure to invalidate finder caches before trying to import generated modules. This is necessary according to the Python documentation: https://docs.python.org/3/library/importlib.html#importlib.invalidate_caches

This fixes a hard-to-reproduce bug that Python would be unable to find just-generated lextab.py if mtime of the current directory did not change from the moment the script was started. This could e.g. be the case if one has second-precision timestamps and removes the generated file just before starting the build, e.g.:

$ rm pycparser/lextab.py; python -m build -nw

It could also be reproduced easier by doing something like:

$ cd pycparser
$ touch .; python -B _build_tables.py
Traceback (most recent call last):
  File "/var/tmp/pycparser/pycparser/_build_tables.py", line 38, in <module>
    import lextab
ModuleNotFoundError: No module named 'lextab'

This is because the first command (rm or touch) updates the mtime of the directory to the current time. If the script is run fast enough, it manages to scan the directory and then write the new lextab.py within the same second. As a result, mtime of the directory after writing the new file is the same as when the script was started, finder does not invalidate the cache and assumes that lextab.py does not exist since it did not exist when the directory was scanned earlier.

This potentially fixes #493.

It was originally reported on https://bugs.gentoo.org/701878. Thanks to Gary E. Miller for patience in reproducing the problem and proxy-debugging it for me, as well as testing the final patch before submission.

[eliben]

Thanks!

[mgorny]

Thanks!