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.