gh-118761: Optimise import time for shlex

[AA-Turner]

This PR achieves a 33x improvement in import time for the shlex module and a 4-12x improvement in performance for shlex.quote.

Current:

import shlex: cumulative time mean: 9065.967 µs median: 9092.000 µs stdev: 282.015 min: 8587 max: 9556 

This PR:

import shlex: cumulative time mean: 274.300 µs median: 275.000 µs stdev: 8.860 min: 259 max: 289 

• Issue: Improve import time of various stdlib modules #118761

[JelleZijlstra]

I'm not sure this is practically useful, since pretty much anything you'd do within the module will trigger the imports. Also, importing sys and io is practically free since they're imported at startup.

[AA-Turner]

True, this one is more marginal. quote and join will only import re, whereas split and shlex will import collections/os.path. Where shlex is used in the stdlib, it's often only one of these two groups, so I think it could still have benefits?

Edit: quote and join now no longer import re

A