Add improve and add tests for FASTA reading.

[alope107]

• Add pytest-mock dependencies and regenerate requirements files. Please make sure you update your environment to pick up this new dependency!
• Renamed local_io.py to fasta.py.
• Fixed bug in read modes for gzipped FASTAs.
• Add fixtures for handling FASTA files in tests. Add tests for happy-path and empty FASTA reading.
• Allow read_fasta to work with str or Path objects.
• Have read_fasta raise a FileContentsError instead of failing an assertion when an empty FASTA is encountered.
• Moved some documentation from README.md to CONTRIBUTING.md.