Install blocked by old version of `pip`

The `install` hook fails due to an old version of `pip`.

When installing "fabric" via pip, the dependency "cryptography" fails with a message stating the version of pip is too old and a traceback as follows: [0]

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/var/lib/pip/pip-build-y4ldt5j2/cryptography/setup.py", line 14, in <module>
        from setuptools_rust import RustExtension
    ModuleNotFoundError: No module named 'setuptools_rust'

The `install` hook succeeds when the modification for bug 1968366 is applied alongside an upgrade of pip as follows:
$ python -m pip install --upgrade pip

If possible, the charm needs to ensure the supported version of pip is available for the dependencies it intends to install during the `install` hook.

[0]: https://pastebin.canonical.com/p/f4YtDSB546/