WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7f5a8c2bc650>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/numpy/

This worked for me.

bash
sudo apt-get update
sudo apt-get upgrade
sudo apt install python3-pip

https://github.com/microsoft/WSL/issues/4020