Update package configuration and automate maintenance actions

[bdewilde]

Description

• adds automated GitHub workflows for building and testing the package, linting and formatting, publishing new releases to PyPi, and building documentation (and removes the old Travis configuration)
• improves and tidies package configuration: bounds some dependencies, moves source code into a top-level src directory
• improves and configures dev tooling: adds a makefile with common commands, adds flake8 for linting, sets default tool configurations in setup and pyproject files
• uses the above tools and actions to significantly improve code consistency and reduce code smell throughout the code base

TODO:

• [x] update package documentation, especially the contributing guide
• [ ] really get a handle on documentation: will readthedocs work? does the GitHub action do what we think??

Motivation and Context

I've had limited time to maintain and improve textacy for the past year, and much of the available time was spent on reactive / rote maintenance. Taking advantage of newer tools to reduce the maintenance burden will free up time for more meaningful improvements.

How Has This Been Tested?

All the tests run automatically :)

Types of changes

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

Checklist:

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