gmail API : ModuleNotFoundError: No module named 'google'

While running the quickstart.py of Gmail, I get the following error on macOS.

Traceback (most recent call last):
  File "quickstart.py", line 4, in <module>
    from googleapiclient.discovery import build
  File "/Users/<user>/Documents/venv/bin/googleapiclient/discovery.py", line 49, in <module>
    import google.api_core.client_options
ModuleNotFoundError: No module named 'google'

These are the installed Google libraries.

$ pip freeze | grep google
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. A future version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support
google-api-core==1.16.0
google-api-python-client==1.8.0
google-auth==1.12.0
google-auth-httplib2==0.0.3
google-auth-oauthlib==0.4.1
google-cloud-bigquery==1.24.0
google-cloud-core==1.3.0
google-resumable-media==0.5.0
googleapis-common-protos==1.51.0

from gmail API : ModuleNotFoundError: No module named 'google'