0.29.3

A full list of commits for 0.29.3 can be found at:

https://github.com/geopython/OWSLib/commits/0.29.3

• new readthedocs configuration
• pre-render doc notebooks
• fix WMS 1.1.1 expression bug
• removed unused lxml code
• ensure schema_key is never None when reading namespaces
• fix WFS2 POST attribute name
• silence FutureWarning when passing XML etree object to filter
• update CI (drops Python 3.7)
• fix WPS logging DEBUG statements
• fix CRS for non-int code by URL