Optimize some repo metastuff and prepare v1.9.0 (python-distro#364)

* Update copyright dates

* Add build dependency to dev-requirements.txt

* Add src/*-egg-info/ to .gitignore

* Add missing Python classifiers

* Remove gitter link from README.md

Our gitter channel doesn't exist anymore anyway.

* Make CONTRIBUTORS.md relevant

* Update CHANGELOG.md with latest unreleased changes

* Release v1.9.0

* Fix review comments

* Add .readthedocs.yaml config to fix docs deployment

Release 1.8.0

Release 1.7.0

Fix release date for 1.6.0

Merge pull request python-distro#258 from nir0s/v1.5.0

Prepare release v1.5.0

Release v1.3.0

Release v1.3.0

Update version to 1.2.0

Update CHANGES