New install package name in Ubuntu 20.04

[Realtime]

On Ubuntu 20.04 the name of the package python-pymysql has changed to python3-pymysql. This should be mentioned in the manual.

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [ ] This doc is inaccurate in this way: ______
- [X] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

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-----------------------------------
Release: on 2020-10-27 10:26:10
SHA: 8635ebf9ad6989f4f4ec20353bb22effe74f28ea
Source: https://opendev.org/openstack/openstack-manuals/src/doc/install-guide/source/environment-sql-database-ubuntu.rst
URL: https://docs.openstack.org/install-guide/environment-sql-database-ubuntu.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/761722

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/761722
Committed: https://git.openstack.org/cgit/openstack/openstack-manuals/commit/?id=dbd8d24b209b7d3d2da77261d798ff7f5a5df19b
Submitter: Zuul
Branch: master

commit dbd8d24b209b7d3d2da77261d798ff7f5a5df19b
Author: chenxing <email address hidden>
Date: Fri Nov 6 18:17:05 2020 +0800

    Update python-pymysql to python3-pymysql for Ubuntu 20.04

    Change-Id: Ibe86bc94ce1a67457e5609ab750ce343308f61dd
    Closes-Bug: #1902920