text: Title: Update Python version in Dockerfile-s
  Body: **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:
  
  - https://github.com/quantumlib/Cirq/blob/74cee7e8e8787ab56160124b3a13834550e31cc1/Dockerfile#L1
  - https://github.com/quantumlib/Cirq/blob/74cee7e8e8787ab56160124b3a13834550e31cc1/dev_tools/docs/Dockerfile#L29
  - https://github.com/quantumlib/Cirq/blob/74cee7e8e8787ab56160124b3a13834550e31cc1/dev_tools/pr_monitor/Dockerfile#L26
  
  **TODO**
  
  Check if all of these Dockerfile-s are in use then update their Python versions to 3.9.