drop Python 2.7 support

[orbeckst]

CI becomes difficult to support for Python 2.7 (see PR #151 ) because there are no conda packages for MDA 1.1.1 and MDA 2.0 does not support Python 2 anyway.

Given PMDA's current experimental focus I would like drop Python 2.7 immediately to reduce the maintenance/support burden.

[orbeckst]

@VOD555 @yuxuanzhuang do you have any opinions on dropping Python 2.7?

I can do the drop along PR #151 because I cannot really test for Python 2.7 in the GH actions framework without huge pain.

[VOD555]

I'm ok with dropping 2.7 in 0.4.

[yuxuanzhuang]

SGTM.

[orbeckst]

Ok, we're ditching Python 2.7 asap.

[orbeckst]

PRs welcome ;-).