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

先从 Modules/posixmodule.c 中链接行附近的 Windows 路径处理代码开始，然后使用报告中的 os.path.abspath 命令重现该行为。确认如何获取驱动器号，并将 Windows 绝对路径中的驱动器号大小写一致且相关行为由测试覆盖定义为完成。