Drop support for Python 3.6

[ccordoba12]

Python 3.6 will reach end-of-life (EOL) in two weeks and we have several blockers due to us still supporting that old version:

• PR: Override RichJupyterWidget._get_color() because we use a custom style (IPython console) #16957 (it's much easier to deal with those changes by simply requiring 3.7+)
• Update Python base version to 3.7+ python-lsp/python-lsp-server#130 (this is required to add new functionality to the PyLSP).

Since the 3.6 EOL will happen in the middle of our 5.2 cycle, I plan to include this in 5.2.2. Fortunately, I haven't seen people opening issues with Python 3.6 in a long time.