Continue tidying pkg and code

[bdewilde]

Description

• updates package configuration: adds a mypy config to reduce output noisiness, tightens up setup for finding packages
• standardizes error messages on common error cases by way of a new errors.py top-level module
• replaces string formatting with f-strings throughout most of the code base
• parametrizes and updates many tests using pytest

Motivation and Context

Just continuing to clean out the cobwebs. textacy needed some love!

How Has This Been Tested?

All tests pass!

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation, and I have updated it accordingly.