Minor CI and typechecking updates

[polsys]

• Bump version number to 1.2.1
• CI updates:
  • Drop 3.7 from unit testing
  • Run integration tests on all PRs
  • Try packaging before typechecking (as the latter is more likely to fail)
• Typechecking:
  • Add some .item() calls where ndarray was implicitly compared to float
  • Add one # type: ignore where the type checker gets confused (sees ndarray access to produce Any)
  • Remove one # type: ignore that mypy now handles correctly