Transition from testdir to pytester

[jeffwright13]

According to the API docs, as of pytest 6.2, the use of testdir is discouraged in favor of the new pytester instance ("New code should avoid using testdir in favor of pytester"). This project should consider updating to comply with the new recommendation.

I grepped my local copy of this repo, and I believe it only references the old 'testdir instance in the file "test_{{cookiecutter.module_name}}.py".

I may be able to make this change myself and issue a PR at some point in the near future, when my knowledge of Pytest is a bit more complete. :-)