Begin resolving type annotation issues

[kurtmckee]

Pull Request check-list

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

Description of change

When mypy is run with its current settings in .mypy.ini, there are just short of 1,000 warnings and failures. This PR begins the work of fixing these mypy failures.

• A new CI job, test-type-annotations, is introduced. It is added to the needs requirements for the test suite matrix.

• The mypy settings are migrated from .mypy.ini to pyproject.toml.

  This is desirable because in the INI config file doesn't support a list of modules to override; the TOML format supports a compact list of globbed modules in the tool.mypy.overrides section, which is easier to read and maintain.

• An initial batch of files' type annotations are fixed.

There are still many, many type annotation issues to address, but this initial work introduces the testing infrastructure to support ongoing improvements and fixes.

[mkmkme]

Thanks. I'm currently handling it in #265. I'll check your PR properly once my PR is merged and we can continue from there.