Dehinter

- remove Python 3.6 support (this was eliminated in our fontTools dependency)
- update fonttools dependency to v4.28.1
- update GitHub Actions CI workflow to include Python 3.10 testing
- update GitHub Actions workflows to use cPython 3.10 runner by default

- add a `dehinter.font.dehint` function to be used by programs that import dehinter as a module
- add `--quiet` flag to suppress standard output reporting
- update fonttools dependency to v4.22.1

- add support for cvar table removal in variable fonts (backward incompatible change)
- add new `--keep-cvar` option to toggle cvar table removal off
- modify default gasp approach to add a gasp table when it is not available in a font (backward incompatible change)
- add new source formatting Makefile targets
- add isort package to setup.py extras_requires dev dependencies
- source import statement formatting
- update fonttools dependency to v4.22.0
- update GitHub Actions workflows to use cPython 3.9 (from cPython 3.8)

- dependency update patch
- update fonttools dependency to v4.17.1

- add cPython 3.9 interpreter testing
- add CodeQL static source testing
- update fonttools dependency to v4.16.1

- refactor string formatting to f-strings
- added GitHub Action CI workflows
- removed Travis CI testing
- removed Appveyor CI testing