Configure pytest with [tool.coverage.report] exclusions

[nathanjmcdougall]

Motivation
Some lines of code we expect to be impossible to reach, e.g. ones with assert_never. But code coverage will degrade because of these lines. You can configure coverage to ignore then with [tool.coverage.report].exclude_also. For example:

[tool.coverage.report]
exclude_also  = [
  "if TYPE_CHECKING:",
  "raise AssertionError",
  "raise NotImplementedError",
  "assert_never(.*)",
  "class .*\\bProtocol\\):",
  "@(abc\\.)?abstractmethod",
]

Summary of desired enhancement
Add logic to the usethis pytest command which automatically adds sensible excludes.

[nathanjmcdougall]

Duplicate of #159