Get CI working again

[guyer]

• [x] Something is borked with the mamba solver
• [x] Newer opcodes break Variable representations
• [x] PyTrilinos only installs in Python 3.7, which is fragile
• [x] Documentation build stalls on HTML

    File "/home/vsts/work/1/s/documentation/_themes/nist/basic_layout.html", line 97, in template
      <link rel="stylesheet" href="{{ pathto('_static/' + style, 1) }}" type="text/css" />
      ^^^^^^^^^^^^^^^^^^^^^^^^^
  jinja2.exceptions.UndefinedError: 'style' is undefined

One consequence of this PR is that tests are run on latest Python 3 that can be installed, instead of hard-wiring 3.10 or 3.7.