pipenv: update pytest to 5.x; remove langcodes

pytest has been pinned to the 4.x series to work around a test import
package mangling problem with citeproc_styles. Now that pytest.ini
explicitly lists test files, this seems to no longer be a problem and
pytest can be updated to the most recent version.

Also re-locked Pipfile.lock with updated dependencies (only minor
changes).

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