Improving test coverage

This issue gathers some areas where we need to improve our test coverage.

You can view pyo3's test coverage online here or locally by following the steps here (do this when writing tests, so you can check them).

In particular (but not limited to) these areas need improvement:

• buffer.rs https://github.com/PyO3/pyo3/pull/2462
• pycell.rs
• types/function.rs https://github.com/PyO3/pyo3/pull/2462
• types/slice.rs https://github.com/PyO3/pyo3/pull/2462
• once_cell

Please keep in mind to write meaningful tests - do not just write tests that take the happy path, but also tests that take error paths or corner cases.