_build_tables: Invalidate cache before importing generated modules (#494

)

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.

pycparser/_build_tables.py

@@ -13,6 +13,7 @@
 # Insert '.' and '..' as first entries to the search path for modules.
 # Restricted environments like embeddable python do not include the
 # current working directory on startup.
+import importlib
 import sys
 sys.path[0:0] = ['.', '..']
 
@@ -32,6 +33,8 @@
 
 # Load to compile into .pyc
 #
+importlib.invalidate_caches()
+
 import lextab
 import yacctab
 import c_ast