Use "ubuntu-latest" for GitHub Actions CI

[rahulporuri]

This PR updates GitHub Actions CI to use ubuntu-latest instead of older ubuntu-* versions. This is possible now as #360 has been resolved and we no longer need to pin ubuntu to older versions.

[rahulporuri]

Most of the test failures end in this way -

...

    import wx
  File "/home/runner/.edm/envs/enable-test-3.6-wx/lib/python3.6/site-packages/wx/__init__.py", line 12, in <module>
    __version__ = wx.__version__.VERSION_STRING
AttributeError: module 'wx' has no attribute '__version__'

This can be replicated offline

ModuleNotFoundError: No module named 'wx.version.VERSION_STRING'; 'wx.version' is not a package

Note that the version I was using locally was 4.1.1 - whereas we pin the version to < 4.1 in traitsui and pyface.