test compatibility with MDAnalysis 2.0.0

[orbeckst]

Expected behaviour

pmda works with MDAnalysis 2.0.0 (or at least only installs MDA < 2.0 if it's not compatible)

Actual behaviour

I don't know — needs to be tested.

TODO

• test against MDA 2.x (possibly when updating CI MAINT: Switch from Travis to Github actions as CI provider #146 ) and report if anything is broken
• possibly fix things so that it works with 2.x
• think about deprecating MDA 1.x dependence and make a final release fully compatible with 1.x
• follow NEP29 as does MDA from 2.1 onwards