gh-101144: Allow open and read_text encoding to be positional. (#101145)

gh-101144: Allow open and read_text encoding to be positional. (#101145)

The zipfile.Path open() and read_text() encoding parameter can be supplied as a positional argument without causing a TypeError again. 3.10.0b1 included a regression that made it keyword only.

Documentation update included as users writing code to be compatible with a wide range of versions will need to consider this for some time.