Use environment variable TYPEGUARD_DISABLE to disable typeguard.

[tage64]

Changes

Setting the environment variable TYPEGUARD_DISABLE disables Typeguard and has the same effect as running Python with the -O flag.

I wanted to compile my Python program with Nuitka which removes the source code so that the inspect module doesn't work. However, I did not want to disable all assertians with the -O flag.

Checklist

• [X] You've added tests (in tests/) added which would fail without your patch
• [X] You've updated the documentation (in docs/, in case of behavior changes or new features)
• [X] You've added a new changelog entry (in docs/versionhistory.rst).

[coveralls]

coverage: 94.458% (-0.05%) from 94.511% when pulling 859a611d82d882037894447c8dc5107903092b0b on tage64:master into b6a7e4387c30a9f7d635712157c889eb073c1ea3 on agronholm:master.