docs: Clarify supported Python versions in README.

[2400032703kotinijeswanth]

Clarify the supported Python versions for Zulip Terminal in the README.

Previously the documentation mentioned "python 3.6+ required for running",
which is misleading based on the currently tested versions.

This change:

• Updates the virtualenv instructions to state that Zulip Terminal currently
  supports Python 3.8–3.11.
• Updates the Pipenv example from --python 3.6 to --python 3.10, and notes
  that any supported 3.8–3.11 version can be used.

Fixes: #1557

[zulipbot]

Hello @2400032703kotinijeswanth, it seems like you have referenced #1557 in your pull request description, but you have not referenced them in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #1557..

An example of a correctly-formatted commit:

commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51.

To learn how to write a great commit message, please refer to our guide.