Inconsistent drive letter case on Windows

Bug report + suggested fix

The os.path.abspath method on windows is inconsistent when pre-pending the drive letter as it can be either upper or lowercase.

It is not entirely clear from Microsoft's documentation (https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getfullpathnamew) when this happens exactly, but, a simple way of re-producing the issue is by changing directory and specifying the opposite case letter in the call

C:\Users\kieran>python -c "import os; print(os.path.abspath('.'))"
C:\Users\kieran

C:\Users\kieran>cd c:\Users\kieran\Projects

c:\Users\kieran\Projects>python -c "import os; print(os.path.abspath('.'))"
c:\Users\kieran\Projects

This is very frustrating when trying to compare paths, and, sometimes different tools in the same development environments will run in such a way that they change their drive letter case, leading to issues between those tools or differences inside tests.

I would like to suggest that we enforce the lowercase driver letters by making a small change to https://github.com/python/cpython/blob/0fe61d0838218b18bcff4bf434beba3e8dbcc42b/Modules/posixmodule.c#L4236-L4274

If we were to include an iteration (line 4266) of the buffered path that lowercased the chars up to the colon (guaranteed to be set), this would ensure consistent drive letters.

Your environment

Python 3.9.6
Microsoft Windows 10 Pro - 10.0.19044 Build 19044
x64-based PC

Start with the Windows path-handling code in Modules/posixmodule.c around the linked lines, then reproduce the behavior using the os.path.abspath commands in the report. Confirm how the drive letter is obtained and define done as consistent drive-letter casing in Windows absolute paths, with the relevant behavior covered by tests.