Fall back to cpu_count when psutil is unavailable at build time

[OuariachiRafik]

Problem

pip install gfootball can fail with:

ModuleNotFoundError: No module named 'psutil'
error: Google Research Football compilation failed

gfootball/build_game_engine.sh runs an inline Python snippet that imports
psutil to size compilation parallelism. psutil is a runtime dependency,
so it is not guaranteed to be present in the build environment — under
PEP 517 build isolation it is absent, and the build aborts before any C++
compilation. Reported in #414.

Fix

Wrap the psutil import in a try/except ImportError and fall back to
multiprocessing.cpu_count() when it is unavailable, with an outer shell
guard falling back to 1. This completes the existing
# TODO: Try importing psutil and if failed fall back to ... in the same file.
Behavior is unchanged when psutil is present.

Testing

Reproduced #414 in a clean environment without psutil in the build environment.
Before: the build failed at running build_ext with the psutil error.
After: the build proceeds past the parallelism step into the C++/cmake stage
(the psutil error no longer occurs).

Fixes #414.

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[OuariachiRafik]

@googlebot I signed the CLA !