Typecheck io, knxip modules and xknx.py

[farmio]

Description

Typechecked io and knxip modules and xknx.py, some code quality and error message improvements.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• The documentation has been adjusted accordingly
• The changes generate no new warnings
• Tests have been added that prove the fix is effective or that the feature works
• The changes are documented in the changelog
• The Homeassistant plugin has been adjusted in case of new config options

[coveralls]

Coverage decreased (-0.2%) to 94.443% when pulling c83559a on farmio:typecheck-io into 7352bcf on XKNX:master.