This PR adds a GitHub action to run unit tests against a (large) matrix of different environments, including operating system, BLAS variant, and installation/build method (pip, conda), copied over from slycot. This is in response to issue #820, where an error was triggered under Ubuntu using the generic BLAS library for NumPy 1.24.0.

The matrix of tests is quite time consuming (about 45 minutes), and so the action is only trigged on pull requests (so if you are using GitHub actions in your own forks, they won't be affected).

In the initial PR, the errors from issue #820 are present (so the tests will fail). In a subsequent push to this PR, I added a "fix" by doing an xfail on that specific test case (and demonstrating one way this set of test might be used).

Also: when a failure occurs in the flatsys test case, information is printed out about the configuration to help in identifying the cases that are failing.