In order to prevent Python from generating the pycache directory, we
need to pass the -B flag [1]. To this end, a new LIT variable is
introduced, %python_b, that expands to python -B.
This is currently only required in Flang as it uses
flang/test/Semantics/common.py that's imported in tests that use
%python. In order to speed-up re-use, this Python module is
pre-compiled by-default and saved in flang/test/Semantics/__pycache__.
This was brought up in a post-commit review in [2]
[1] https://docs.python.org/3/using/cmdline.html#id1
[2] https://reviews.llvm.org/D107041