Attempt to fix windows build by calling libtbx pytest explicitly

[ndevenish]

With cctbx/dxtbx@0cc3c65, the dxtbx dispatcher generation changed slightly. The old pytest override should have been preserved with this entry_point. For some reason this doesn't appear to be working for the windows PR build.

This works around the problem by calling libtbx.python explicitly, bypassing the need for an explicit pytest outer dispatcher. This is done in order to fix the builds now.

[codecov]

Codecov Report

Merging #1791 (1a0276b) into main (8f21749) will not change coverage.
The diff coverage is n/a.

❗ Current head 1a0276b differs from pull request most recent head 13f9f4c. Consider uploading reports for the commit 13f9f4c to get more accurate results

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