perf python: Fix setup.py mypy errors

getenv may return None, so assert it isn't None for CC and srctree
environmental variables required for the script.
Disable an optional warning related to Popen.

Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250311213628.569562-7-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>