kunit: tool: fix newly introduced typechecker errors

After upgrading mypy and pytype from pip, we see 2 new errors when
running ./tools/testing/kunit/run_checks.py.

Error #1: mypy and pytype
They now deduce that importlib.util.spec_from_file_location() can return
None and note that we're not checking for this.

We validate that the arch is valid (i.e. the file exists) beforehand.
Add in an `asssert spec is not None` to appease the checkers.

Error #2: pytype bug https://github.com/google/pytype/issues/1057


It doesn't like `from datetime import datetime`, specifically that a
type shares a name with a module.

We can workaround this by either
* renaming the import or just using `import datetime`
* passing the new `--fix-module-collisions` flag to pytype.

We pick the first option for now because
* the flag is quite new, only in the 2021.11.29 release.
* I'd prefer if people can just run `pytype <file>`

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>