add GROMACS 2023 to test matrix

[orbeckst]

Locally I get some failing tests (see PR #250 and issue #249 ) with gmx 2023 so we need to be testing with it.

[orbeckst]

Plenty of stuff to fix:

• do_dssp was replaced by gmx dssp #249
• fix release test TestRelease.test_release_startswith

FAILED tests/test_tools.py::TestRelease::test_release - AssertionError: assert False
 +  where False = <built-in method startswith of str object at 0x7f0762e63690>(('4', '5', '2016', '2018', '2019', '2020', ...))
 +    where <built-in method startswith of str object at 0x7f0762e63690> = '2023.1-conda_forge'.startswith
 +      where '2023.1-conda_forge' = <gromacs.tools.Release object at 0x7f0762e67310>()
 +        where <gromacs.tools.Release object at 0x7f0762e67310> = gromacs.release
 +    and   ('4', '5', '2016', '2018', '2019', '2020', ...) = <tests.test_tools.TestRelease object at 0x7f0762d2bbd0>.major_releases
FAILED tests/test_tools.py::TestRelease::test_release_startswith - AssertionError: assert False
 +  where False = <bound method Release.startswith of <gromacs.tools.Release object at 0x7f0762e67310>>(('4', '5', '2016', '2018', '2019', '2020', ...))
 +    where <bound method Release.startswith of <gromacs.tools.Release object at 0x7f0762e67310>> = <gromacs.tools.Release object at 0x7f0762e67310>.startswith
 +      where <gromacs.tools.Release object at 0x7f0762e67310> = gromacs.release
 +    and   ('4', '5', '2016', '2018', '2019', '2020', ...) = <tests.test_tools.TestRelease object at 0x7f0762d2a150>.major_releases
ERROR tests/test_tools.py::test_tools_help[do_dssp] - AttributeError: module 'gromacs' has no attribute 'do_dssp'