Update Python version in Dockerfile-s

[pavoljuhas]

Description of the issue

#6167 raises the minimum Python version to 3.9,
but the following Dockerfile-s derive from images with Python 3.7 or 3.8:

• Cirq/Dockerfile

  Line 1 in 74cee7e

  FROM python:3.8-slim AS cirq_base

• Cirq/dev_tools/docs/Dockerfile

  Line 29 in 74cee7e

  FROM nikolaik/python-nodejs:python3.7-nodejs14

• Cirq/dev_tools/pr_monitor/Dockerfile

  Line 26 in 74cee7e

  FROM python:3.7-slim@sha256:1689dea3fe5e2f230c3b2d0cc18b64e96f8214082c78792c67dbc1bda9b6c67c

TODO

Check if all of these Dockerfile-s are in use then update their Python versions to 3.9.

[pavoljuhas]

cc: @tanujkhattar

[NoureldinYosri]

cirq-sync: accepted to updating the python version for the docker images in use ... we also need to findout which docker images are not being used to drop them

[pavoljuhas]

TODO @pavoljuhas - verify if anyone uses or wants to archive Cirq/dev_tools/docs/Dockerfile

[jackreeceejini]

@NoureldinYosri Sorry did not see that you had accepted this because I had already updated the Python version to 3.9 but I don't know how to find out if they are in use or not though

[pavoljuhas]

... I don't know how to find out if they are in use or not though

@jackreeceejini - thank you for taking this on. I have asked on our internal cirq chat with no response so I submitted #6184 to delete it. Please hold on with changing Cirq/dev_tools/docs/Dockerfile until that PR is resolved, but feel free to update the other Dockerfile-s.