Replace with standard libraries such as pathlib

A lot of the code in certain files, such as `src/commoncode/paths.py` and `src/commoncode/filetype.py` and possible others can be replaced by standard methods from the `pathlib` module. This has a few benefits:

1. less maintenance on your part
2. `pathlib` has seen more testing than your code
3. possible optimisations in `pathlib`