The proposed PR converts os.getgrouplist(), os.initgroups(), os.sendfile() and os.get_terminal_size() to Argument Clinic.

The problems which prevented their conversions before (in issue20170):

1. os.sendfile() had parameter names conflicting with Python keywords. Was solved in issue38378.

2. os.get_terminal_size() has an optional argument without default value. It was solved in a way similar to issue37206.

3. Some functions have platform-specific types of arguments. os.sendfile() has additional parameters. It was solved by using a preprocessor. We need to repeat most of the declaration and docstring, but it is the best that we can have now.

#37206 changed the signature "pop(self, key, default=None, /)" back to "D.pop(k[,d]) -> v", taken from dict.pop.__doc__.  However, the incorrect version that got reverted correctly added the positional-only indicator '/'.  Docstrings with signatures for such cases may need updating.

Unlike most objects with invalid signatures (as seen by inspect.signature), the current docstring for os.get_terminal_size does not have a backup signature for help (and IDLE) to present.  I cannot tell if the patch adds it.

New changeset 2b5603140c09766a7d4e8243a70d7144f684f6f9 by Serhiy Storchaka in branch 'master':
bpo-40178: Convert the remaining os functions to Argument Clinic. (GH-19360)
https://github.com/python/cpython/commit/2b5603140c09766a7d4e8243a70d7144f684f6f9