Pip 19.1.1 and earlier not supported on Python 3.10

[jaraco]

Integration tests try to use old versions of pip that are not compatible with Python 3.10 due to collections.abc aliases removed from collections.

Originally posted by @hroncok in #2558 (comment)

[jaraco]

With some bisection, I determined that Pip 19.2 has the fix so 19.1.1 and earlier will never work on Python 3.10.

etuptools main $ python3.10 -m pip-run 'pip<19.2' -- -m pip --version
Collecting pip<19.2
  Using cached pip-19.1.1-py2.py3-none-any.whl (1.4 MB)
Installing collected packages: pip
Successfully installed pip-19.1.1
ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
pip-run 8.4.2 requires pip>=19.3, but you have pip 19.1.1 which is incompatible.
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/__main__.py", line 16, in <module>
    from pip._internal import main as _main  # isort:skip # noqa
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_internal/__init__.py", line 40, in <module>
    from pip._internal.cli.autocompletion import autocomplete
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_internal/cli/autocompletion.py", line 8, in <module>
    from pip._internal.cli.main_parser import create_main_parser
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_internal/cli/main_parser.py", line 12, in <module>
    from pip._internal.commands import (
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_internal/commands/__init__.py", line 6, in <module>
    from pip._internal.commands.completion import CompletionCommand
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_internal/commands/completion.py", line 6, in <module>
    from pip._internal.cli.base_command import Command
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_internal/cli/base_command.py", line 25, in <module>
    from pip._internal.index import PackageFinder
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_internal/index.py", line 14, in <module>
    from pip._vendor import html5lib, requests, six
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_vendor/html5lib/__init__.py", line 25, in <module>
    from .html5parser import HTMLParser, parse, parseFragment
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_vendor/html5lib/html5parser.py", line 8, in <module>
    from . import _tokenizer
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_vendor/html5lib/_tokenizer.py", line 16, in <module>
    from ._trie import Trie
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_vendor/html5lib/_trie/__init__.py", line 3, in <module>
    from .py import Trie as PyTrie
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_vendor/html5lib/_trie/py.py", line 6, in <module>
    from ._base import Trie as ABCTrie
  File "/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-run-2a9ynblh/pip/_vendor/html5lib/_trie/_base.py", line 3, in <module>
    from collections import Mapping
ImportError: cannot import name 'Mapping' from 'collections' (/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/collections/__init__.py)

[hroncok]

Awesome, thanks!