pip installで発生するImportError: Cannot import name 'webencodings' from 'pip.

前提・実現したいこと

表示されているメッセージに従いpipのupgradeをした後、下記のエラーメッセージと共にパッケージインストールが出来なくなった。
installだけではなく、listなども下記メッセージが出る。
コマンドプロンプトからパッケージをインストール出来るようにしたい。

発生している問題・エラーメッセージ

ImportError: cannot import name 'webencodings' from 'pip._vendor' (C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_vendor\__init__.py)

該当のソースコード

python
1C:\Users\A>python -m pip install python-pptx
2Traceback (most recent call last):
3  File "C:\Program Files (x86)\Python38-32\lib\runpy.py", line 193, in _run_module_as_main
4    return _run_code(code, main_globals, None,
5  File "C:\Program Files (x86)\Python38-32\lib\runpy.py", line 86, in _run_code
6    exec(code, run_globals)
7  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\__main__.py", line 19, in <module>
8    sys.exit(_main())
9  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_internal\cli\main.py", line 73, in main
10    command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
11  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_internal\commands\__init__.py", line 96, in create_command
12    module = importlib.import_module(module_path)
13  File "C:\Program Files (x86)\Python38-32\lib\importlib\__init__.py", line 127, in import_module
14    return _bootstrap._gcd_import(name[level:], package, level)
15  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
16  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
17  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
18  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
19  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
20  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
21  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_internal\commands\install.py", line 24, in <module>
22    from pip._internal.cli.req_command import RequirementCommand
23  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_internal\cli\req_command.py", line 15, in <module>
24    from pip._internal.index.package_finder import PackageFinder
25  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_internal\index\package_finder.py", line 21, in <module>
26    from pip._internal.index.collector import parse_links
27  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_internal\index\collector.py", line 12, in <module>
28    from pip._vendor import html5lib, requests
29  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_vendor\html5lib\__init__.py", line 25, in <module>
30    from .html5parser import HTMLParser, parse, parseFragment
31  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_vendor\html5lib\html5parser.py", line 7, in <module>
32    from . import _inputstream
33  File "C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_vendor\html5lib\_inputstream.py", line 9, in <module>
34    from pip._vendor import webencodings
35ImportError: cannot import name 'webencodings' from 'pip._vendor' (C:\Users\A\AppData\Roaming\Python\Python38\site-packages\pip\_vendor\__init__.py)