Updates for OpenMM 7.6

[dwhswenson]

Most significantly, this fixes a test error on the URL that @sroet suggested I add a test for (thanks @sroet!) by removing the simtk from the URL. I also went ahead and made it so we preferred new 7.6-style imports everywhere else.

[codecov]

Codecov Report

Merging #50 (70bdc3c) into main (5581252) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #50   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           29        29           
  Lines         1202      1204    +2     
=========================================
+ Hits          1202      1204    +2     

Impacted Files	Coverage Δ
paths_cli/wizard/openmm.py	100.00% <100.00%> (ø)

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[dwhswenson]

This is ready for review and comment. I will leave it open for at least 24 hours, merging no earlier than Fri 03 Sep 01:00 GMT (Thu 02 Sep 21:00 my local).

[sroet]

A couple left over old code and missing reasons for skips. I am not too happy about the code duplication on the test files (around the imports), but this is easier to understand than indirect imports, so no favor one way or another (As long as it exists in just 2 files)

[sroet]

Most significantly, this fixes a test error on the URL that @sroet suggested I add a test for (thanks @sroet!)

Glad this test already paid off 😄

[dwhswenson]

Made changes, though not quite the ones you suggested. I intentionally left the commented importorskip to make it very easy to switch back to that code when we drop support for OpenMM < 7.6. I've added a comment to make that clear. Also followed your suggestion to add a message to the skip.

[sroet]

LGTM