Fdiff

- Add cPython 3.9 interpreter unit testing
- Add CodeQL static source testing
- Bump fonttools dependency to v4.16.1
- Bump aiohttp dependency to v3.6.3
- Bump multidict dependency to v5.0.0
- Bump yarl dependency to v1.6.2

- Add type annotations to all Python source files
- Refactor `remote.py` module namedtuple to Py3.6+ style `NamedTuple` derived class with type annotations
- Transition from pytype to mypy as static type checker
- Add GitHub Action static type check configuration
- Refactor import statements with default `isort` formatting
- Bump fonttools dependency to v4.15.0

- update cffi dependency to bug fix release v1.14.3

- Refactor to maintain line length < 90
- Add flake8 linting as part of the CI
- Transition to GitHub Actions CI testing service
- Bump attrs dependency to v20.2.0
- Bump cffi dependency to v1.14.2
- Bump fonttools dependency to v4.14.0
- Bump idna dependency to v2.10
- Bump multidict dependency to v4.7.6
- Bump yarl dependendency to v1.5.1

- Backward incompatible change in the default unified diff approach for large files (fixes 54)
- Transition to the upstream cPython implementation of `difflib.unified_diff` for diff execution
- Remove cPython `difflib` derivative that was distributed with this project
- Eliminate dual license structure with removal of cPython license and transition to Apache License, v2.0 only

- Bump cffi to 1.14.0
- Bump fonttools to 4.6.0
- Bump idna to 2.9
- Bump multidict to 4.7.5
- Bump pycares to 3.1.1
- Bump pycparser to 2.20
- Source formatting (black)
- Add Py3.8 CI testing on Linux
- Add Py3.6 64-bit testing, Py3.8 64-bit testing on Windows
- Use pinned dependency versions in CI testing with tox