Compatibility with python 3.9+

Bugfix related to Issue Hoohm#192 and Pull Request Hoohm#193 . After pandas>1.5 columns cannot be a set. Added and tested the following changes:

• Ensure order of top_cells and aberrant_cells
• Updated docstrings
• Update pytest version to support python 3.9+
• Changes are tested.

What's Changed

• Bugfix/pr 193 columns cannot be set by @Tobiaspk in #1
• Bugfix/pr 193 columns cannot be set by @Tobiaspk in #2

Full Changelog: https://github.com/sail-mskcc/CITE-seq-Count/commits/1.4.5