Unpin pytest in edx-platform

Description

In https://github.com/edx/edx-platform/pull/20728 we are having to pin pytest to < 4.6.0 to work around some errors in `common/lib/xmodule/xmodule/tests/test_resource_templates.py` on pkg_resources calls. Looks like pytest switched from pkg_resources to importlib-metadata here: https://github.com/pytest-dev/pytest/pull/5063/files so I'm guessing it accidentally did some setup for us that we might need to do ourselves now?

Specifically it seems to not know the loader type to use. Here is an example error:

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Reporter

Brian Mesick

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