Add Cython pyx, pxd, pxi files to MANIFEST.in

[bdice]

Description

The release of freud 2.3.0 failed on conda-forge with the PyPI tarball: conda-forge/freud-feedstock#30

The root cause is that freud's MANIFEST.in did not include Cython *.pyx and *.pxd files in the source distribution, meaning that the source distribution did not detect the Cython modules and could not build. Luckily the tarball hosted at http://glotzerlab.engin.umich.edu/Downloads/freud/ included these files, allowing for a build that is still nearly-canonical (building from the PyPI tarball would have been preferable because the PyPI tarball is smaller and consistent with the released PyPI builds).

Motivation and Context

Fixes conda-forge builds for the future.

How Has This Been Tested?

Tested locally by verifying the outputs of python setup.py sdist.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds or improves functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation improvement (updates to user guides, docstrings, or developer docs)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have updated the documentation (if relevant).
• I have added tests that cover my changes (if relevant).
• All new and existing tests passed.
• I have updated the credits.
• I have updated the Changelog.

[b-butler]

LGTM