Minor update, better packaging, docs

v2.2.2 (1/25/2022)

• modify _mie_An_Bn to allocate and return An and Bn
• fix minor packaging issue
• explicitly define encoding when opening files
• explicitly use .readthedocs.yaml to build docs
• use rtd theme for docs
• add docs/requirements
• restrict Jinja2 to 2.11.3 in docs/requirements

v2.2.1 (9/5/2021)

• create pure python packages
• include wheel file
• package as python3 only