chore: drop python 3.6 for cibuildwheel driver

This drops python 3.6 for cibuildwheel.
Building python 3.6 packages is still supported.

docs/contributing.md

@@ -94,7 +94,6 @@ This has been moved to using docker, so you only need the following instructions
 The dependency update script in the next section requires multiple python versions installed. One way to do this is to use `pyenv`:
 
 ```bash
-pyenv install 3.6.11
 pyenv install 3.7.8
 # Optionally add 3.8 and make it the local version;
 # otherwise assuming 3.8+ already is your current python version
@@ -103,7 +102,6 @@ pyenv install 3.7.8
 Then, you need to make the required virtual environments:
 
 ```bash
-$(pyenv prefix 3.6.11)/bin/python -m venv env36
 $(pyenv prefix 3.7.8)/bin/python -m venv env37
 ```