Ufolint

- add: validation for the presence of contents.plist file in UFO source directories
- fix: refactored deprecated fontTools.ufoLib.UFOReader.formatVersion property to new formatVersionTuple
- updated fontTools to v4.18.0 and required 4.18.0+ in this project to support the new contents.plist validation that was added in this release

- fix: contents.plist Path object must be cast to a str to address fontTools ufoLib expectations, this addresses a ufoLib exception raised on contents.plist validation introduced in v1.1.0 release

- new validation: confirm that all glif files are listed in contents.plist (82)
- update fonttools dependency to v4.17.1
- add Python 3.9 classifier in setup.py
- add pull request trigger for GitHub Actions CI testing

- dependency update patch
- bump fonttools to v4.17.0
- bump pytz to v2020.4

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

- refactor to achieve line lengths < 90
- convert to GitHub Actions CI testing service
- upgrade appdirs dependency to v1.4.4
- upgrade fonttools dependency to v4.14.0
- upgrade pytz dependency to v2020.1
- upgrade six dependency to 1.15.0