WIP: bpo-39939: Use removeprefix() and removesuffix()

[vstinner]

DON'T MERGE THIS PR! PR only written to see how the PEP 616 could be
used in the stdlib.

https://bugs.python.org/issue39939

[vstinner]

I created this PR using my pyreplace.py tool: https://github.com/vstinner/misc/blob/master/bin/pyreplace.py

Replace if x.startswith(pattern): x = x[len(pattern):] and if x.endswith(pattern): x = x[:-len(pattern)]:

pyreplace.py \
  'if <expr>.startswith(<expr>): <expr> = <expr>[<regex:[^:]+>:]' \
  '<1> = <1>.removeprefix(<2>)' \
  $(find -name "*.py")

pyreplace.py \
  'if <expr>.endswith(<expr>): <expr> = <expr>[:-<regex:[^]]+>]' \
  '<1> = <1>.removesuffix(<2>)' \
  $(find -name "*.py")

Then I reverted changes which produced invalid code.

I also used an inaccurate regex to discover a few more cases:

pyreplace.py \
  'if <expr>.endswith(<expr>): <regex:.*><expr>[:-<regex:[^]]+>]' \
  '<1> = <1>.removesuffix(<2>)' \
  $(find -name "*.py")

pyreplace.py \
  'if <expr>.startswith(<expr>): <regex:.*><expr>[<regex:[^:]+>:]' \
  '<1> = <1>.removeprefix(<2>)' \
  $(find -name "*.py")

I picked the very few changes which are valid, and revert all other changes.

To finish, I used examples of the PEP 616.

[sweeneyde]

I just made PR at vstinner#3 with a few more examples.