refactor: move some tests inside the lib

[davidhewitt]

This is a cleanup of various freestanding test files as a slight declutter of the codebase:

• Inline the following unit-test-like suites:

  • test_datetime.rs into src/types/datetime.rs
  • test_serde.rs into src/conversions/serde.rs
  • test_string.rs into src/conversions/std/string.rs (rewriting the one test to not need a #[pyfunction])

• Remove the following test suites:

  • test_anyhow.rs (unit tests already exist, testing a PyErr return from #[pyfunction] seems orthogonal to the feature)
  • test_bytes.rs (preserve one unit test for Py::as_bytes in src/conversions/bytes.rs

(I'm hoping that if I do a bit of this test suite decluttering every now and again we can have CI running more efficiently...)

Also fixes #5344 at the same time.