Don't have mypy failure fail the whole tox run

Other environments would still be run even after mypy has failed,
but to avoid having tox runs be unnecessarily inconsistent with the
mypy step in the pythonpackage.yml CI workflow, and also because
GitPython is not currently expected to pass mypy checks, this keeps
mypy errors from causing the whole tox run to be reported as
failed.

Author: EliahKagan

tox.ini

@@ -18,6 +18,7 @@ commands = pre-commit run --all-files
 description = Typecheck with mypy
 basepython = py39
 commands = mypy -p git
+ignore_outcome = true
 
 [testenv:black]
 description = Check style with black