examples/usb/acm_serial does not meet timing requirements on FomuPVT (Max 46.82 MHz)

[umarcor]

Currently, the CI in this repo seems not to be executed. Workflows are triggered, but tox runs no commands. The first commit in this PR is a quick fix for that, which enables tests on py37, py38 and py39.

The second commit shows an issue when trying to build example usb/acm_serial for Fomu (PVT version). Timing is not met, but it's really close:

ERROR: Max frequency for clock              'car_clk': 46.82 MHz (FAIL at 48.00 MHz)

Was this tested before? Is the example expected to work on Fomu?

[umarcor]

I rebased this PR. It seems that some synthesis/PnR steps are executed, but it cannot find the bitstream: https://github.com/umarcor/luna/runs/3037219991?check_suite_focus=true#step:4:315.

[miek]

The CI not running was fixed in 2a0116c and the timings were improved for iCE40 in #133. The acm_serial example on FomuPVT builds successfully for me now, so I'm closing this. Feel free to open an issue if you have any more trouble with it though, thanks!