Move documentation dependencies to extras_require, add testing

[betaBison]

Thanks @rahulbhadani for maintaining bagpy over the years.

I'd like to add a dependency on bagpy in one of my projects, but the dependency on a pinned Sphinx version is incompatible with my other project dependencies.

This pull request:

• moves Sphinx documentation dependencies to extras_require in setup.py. This means that normal users can still install using pip install bagpy, but development users can install with pip install -e .[dev] to add all of the development dependencies in the extra packages to be able to build the docs.
• adds simple tests for bagreader.py to verify compatibility
• adds GitHub action for pip installing and pytests for Python 3.6, 3.7.5, and 3.10 on the latest Ubuntu, Ubuntu 18.04 and the latest MacOS. All tests are passing on my branch.
• adds GitHub action for building docs with Python 3.6, 3.7.5, and 3.10 on the latest Ubuntu, Ubuntu 18.04 and the latest MacOS. All documentation build tests are passing on my branch (although there are warnings when building the docs and I'm not confident I know what I should be looking for in the built html).
• increments stated compatibility from 3.5 to 3.6 (the tests seemed to indicate that the pinned matplotlib>=3.3.1 was incompatible with Python 3.5)

Note: I didn't know how to test the .Travis.yml before creating a pull request, so please let me know how to verify the install update I made to it.

[betaBison]

fixes #30

[betaBison]

I tried (and failed) to fix Travis CI. You can see the documentation builds passing with the new build-docs workflow here: https://github.com/betaBison/bagpy/runs/7925454189?check_suite_focus=true

[betaBison]

Anything I can do to push this pull request through @rahulbhadani ?