gh-117352: Handle /:/ for ntpath.isabs

[nineteendo]

Benchmark:

# test.sh
python -m timeit -s "import before.ntpath""before.ntpath.isabs('C:/foo')"
python -m timeit -s "import after.ntpath""after.ntpath.isabs('C:/foo')"

500000 loops, best of 5: 398 nsec per loop # before
500000 loops, best of 5: 469 nsec per loop # after
# -> 1.18x slower

• Issue: Handle /:/ for ntpath.isabs #117352

[nineteendo]

I've reverted the slicing. The speed decrease of str.startswith() should be fixed soon.

[zooba]

Why would we handle /: differently from any other invalid drive character? Is there some important reason to do this as opposed to more thoroughly checking for all invalid paths?

[nineteendo]

#101363 (comment):

WinAPI GetFullPathNameW() will accept any character in the 16-bit BMP as a drive 'letter', except for null, slash, and backslash.

You could restrict it further, but that would make the code more complicated.