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

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

[coveralls]

coverage: 94.458% (-0.05%) from 94.511%
when pulling 859a611 on tage64:master
into b6a7e43 on agronholm:master.