0.28.0

A full list of commits for 0.28.0 can be found at:

https://github.com/geopython/OWSLib/commits/0.28.0

• core: remove Python 3.6 from CI matrix (@kalxas)
• OGC API: add support for OGC API - Features - Part 4: transactions
• OGC API: add support for OGC API - Maps
• OGC API: fix OGC API - Processes process list filtering
• fix WMTS resolution (@dvbmgr)
• WFS: fix filter parsing (@huard)
• OGC API: use yaml.safe_load for YAML parsing for OpenAPI handling (@sebastic)
• FGDC: fix contact parsing (@pvgenuchten)