Implicit fallthroughs prevent compiler warning enablement

[r-barnes]

Describe the bug
There are a number of places like this where the code does an implicit fallthrough.

Could these please be marked with __attribute__((fallthrough)) (for C code) or [[fallthrough]] (for C++17 and above code)? That enables the use of -Wimplicit-fallthrough which requires that all fallthroughs be explicit, reducing potential error surfaces.

To Reproduce
To assist in reproducing the bug, please include the following:

1. Command/code being executed: N/A
2. Python version and OS: N/A
3. pip version: master
4. Output of pip list or conda list: N/A

Expected behavior
Fallthroughs are marked explicitly.

Additional context
Add any other context about the problem here.

[rdbisme]

Hello @r-barnes, thanks.

Wanna give it a go with a contribution? :)