kunit: tool: undo type subscripts for subprocess.Popen

Writing `subprocess.Popen[str]` requires python 3.9+.
kunit.py has an assertion that the python version is 3.7+, so we should
try to stay backwards compatible.

This conflicts a bit with commit 1da2e6220e11 ("kunit: tool: fix
pre-existing `mypy --strict` errors and update run_checks.py"), since
mypy complains like so
> kunit_kernel.py:95: error: Missing type parameters for generic type "Popen" [type-arg]

Note: `mypy --strict --python-version 3.7` does not work.

We could annotate each file with comments like
`# mypy: disable-error-code="type-arg"
but then we might still get nudged to break back-compat in other files.

This patch adds a `mypy.ini` file since it seems like the only way to
disable specific error codes for all our files.

Note: run_checks.py doesn't need to specify `--config_file mypy.ini`,
but I think being explicit is better, particularly since most kernel
devs won't be familiar with how mypy works.

Fixes: 695e26030858 ("kunit: tool: add subscripts for type annotations where appropriate")
Reported-by: SeongJae Park <sj@kernel.org>
Link: https://lore.kernel.org/linux-kselftest/20230501171520.138753-1-sj@kernel.org
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Tested-by: SeongJae Park <sj@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>