ModuleNotFoundError: No module named 'typing_extensions'

[kajinamit]

Commit a7b2c4e introduced the usage of typing_extensions in Python < 3.11 but it didn't add the library to requirements.
Due to this now we see MoudleNotFoundError in Python 3.10 .

Example:

Traceback (most recent call last):
  File "/usr/os-testr-env/bin/subunit2html", line 3, in <module>
    from os_testr.subunit2html import main
  File "/usr/os-testr-env/lib/python3.10/site-packages/os_testr/subunit2html.py", line 65, in <module>
    import subunit
  File "/usr/os-testr-env/lib/python3.10/site-packages/subunit/__init__.py", line 125, in <module>
    from testtools import ExtendedToOriginalDecorator, content, content_type
  File "/usr/os-testr-env/lib/python3.10/site-packages/testtools/__init__.py", line 48, in <module>
    from testtools.matchers._impl import Matcher  # noqa: F401
  File "/usr/os-testr-env/lib/python3.10/site-packages/testtools/matchers/__init__.py", line 65, in <module>
    from ._basic import (
  File "/usr/os-testr-env/lib/python3.10/site-packages/testtools/matchers/_basic.py", line 29, in <module>
    from ._higherorder import (
  File "/usr/os-testr-env/lib/python3.10/site-packages/testtools/matchers/_higherorder.py", line 21, in <module>
    from typing_extensions import Unpack
ModuleNotFoundError: No module named 'typing_extensions'