Use Python 3.9.4 in CI and Docker container

[per1234]

The actions/setup-python action used to install Python for use in the CI workflows no longer offers Python 3.8.6,
which caused the CI to fail.

The action should use the same version of Python it's tested with, so I have also updated the action's Docker image to
python:3.9.4.

[github-actions]

Memory usage change @ 651f05f4d4aca30ac359e972c01568f873112d43

Board	flash	%	RAM for global variables	%
arduino:avr:leonardo	💚 -40 - 0	-0.14 - 0.0	💚 -16 - 0	-0.62 - 0.0
arduino:sam:arduino_due_x	0 - 0	0.0 - 0.0	N/A	N/A
arduino:samd:mkrzero	❔ -28 - +84	-0.01 - +0.03	🔺 0 - +32	0.0 - +0.1

Click for full report table

Board	examples/MIDIUSB_clock flash	%	examples/MIDIUSB_clock RAM for global variables	%	examples/MIDIUSB_loop flash	%	examples/MIDIUSB_loop RAM for global variables	%	examples/MIDIUSB_read flash	%	examples/MIDIUSB_read RAM for global variables	%	examples/MIDIUSB_write flash	%	examples/MIDIUSB_write RAM for global variables	%
arduino:avr:leonardo	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	-40	-0.14	-16	-0.62
arduino:sam:arduino_due_x	0	0.0	N/A	N/A	0	0.0	N/A	N/A	0	0.0	N/A	N/A	0	0.0	N/A	N/A
arduino:samd:mkrzero	84	0.03	0	0.0	0	0.0	0	0.0	-28	-0.01	0	0.0	0	0.0	32	0.1

Click for full report CSV

Board,examples/MIDIUSB_clock<br>flash,%,examples/MIDIUSB_clock<br>RAM for global variables,%,examples/MIDIUSB_loop<br>flash,%,examples/MIDIUSB_loop<br>RAM for global variables,%,examples/MIDIUSB_read<br>flash,%,examples/MIDIUSB_read<br>RAM for global variables,%,examples/MIDIUSB_write<br>flash,%,examples/MIDIUSB_write<br>RAM for global variables,%
arduino:avr:leonardo,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,-40,-0.14,-16,-0.62
arduino:sam:arduino_due_x,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A
arduino:samd:mkrzero,84,0.03,0,0.0,0,0.0,0,0.0,-28,-0.01,0,0.0,0,0.0,32,0.1