noarch python>=3.6?

This issue arose as an observation made in this PR: https://github.com/conda-forge/wcwidth-feedstock/pull/16.

### The issue
We are dropping python2.7 support, but our noarch packages are by default declaring themselves compatible with python2.7. This leads to a problematic situation: If we do not keep maintaining python2.7 compatibility of our noarch packages, we will retroactively break some legacy python2 environments.
If we keep maintaining python2.7 compatibility, we burden ourselves with unreasonable maintenance.

### A possible solution
All newly built noarch packages declare compatibility with python>=3.6 and thus won't be pulled into legacy python2.7 environments.

I am unsure how a good technical implementation would look like.

Ping @isuruf, @ocefpaf, @CJ-Wright, @beckermr