Replace variable PY_SRC_ROOT by PY_CHECK_ROOTS. The name PY_SRC_ROOT
was a bad choice, given that it isn't immediately obvious that it a)
can contain multiple root locations to be checked, and that it b)
specifically concerns static type checking.
As of this commit, it's possible to limit the type checking scope
with PY_CHECK_ROOTS as in
PY_CHECK_ROOTS="src/python/jw/pkg/CmdBase.py src/python/jw/lib" \
make check
Signed-off-by: Jan Lindemann <jan@janware.com>