Pass through NUMBER_OF_PROCESSORS on Windows (#2647)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: gaborbernat

docs/changelog/2629.bugfix.rst

@@ -0,0 +1,2 @@
+Pass through ``NUMBER_OF_PROCESSORS`` on Windows as is needed for ``multiprocessing.cpu_count`` -
+by :user:`gaborbernat`.

src/tox/tox_env/python/api.py

@@ -94,6 +94,7 @@ def _default_pass_env(self) -> list[str]:
                     "SYSTEMROOT",  # needed for python's crypto module
                     "COMSPEC",  # needed for distutils cygwin compiler
                     "PROCESSOR_ARCHITECTURE",  # platform.machine()
+                    "NUMBER_OF_PROCESSORS",  # multiprocessing.cpu_count()
                 ],
             )
         env.extend(["REQUESTS_CA_BUNDLE"])

tests/session/cmd/test_show_config.py

@@ -100,7 +100,7 @@ def test_pass_env_config_default(tox_project: ToxProjectCreator, stdout_is_atty:
     expected = (
         (["COMSPEC"] if is_win else [])
         + ["CURL_CA_BUNDLE", "LANG", "LANGUAGE", "LD_LIBRARY_PATH"]
-        + (["MSYSTEM", "PATHEXT"] if is_win else [])
+        + (["MSYSTEM", "NUMBER_OF_PROCESSORS", "PATHEXT"] if is_win else [])
         + ["PIP_*"]
         + (["PROCESSOR_ARCHITECTURE"] if is_win else [])
         + (["PROGRAMDATA"] if is_win else [])