Wrap convenience import of machine.PWM inside a try/except

[kyoungmeyer]

The stm32 port of micropython doesn't include the machine.PWM module, this will make the convenience import optional.

Without this patch, adding the device with:

import belay

device = belay.Device("/dev/ttyACM0")

produces the error:

Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    device = belay.Device("/dev/ttyACM0")
  File "/home/kyoungmeyer/Code/IoT-pyboard/belay/belay/device.py", line 267, in __init__
    self._exec_snippet("convenience_imports_micropython")
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/kyoungmeyer/Code/IoT-pyboard/belay/belay/device.py", line 339, in _exec_snippet
    return self("\n".join(snippets), **kwargs)
  File "/home/kyoungmeyer/Code/IoT-pyboard/belay/belay/device.py", line 562, in __call__
    self._board.exec(cmd, data_consumer=data_consumer)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/kyoungmeyer/Code/IoT-pyboard/belay/belay/pyboard.py", line 601, in exec
    raise PyboardException(ret_err.decode())
belay.pyboard.PyboardException:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ImportError: can't import name PWM

This patch fixes the error. Tested on a Pyboard v1.1 with MicroPython 1.27.0.

[BrianPugh]

thanks! i'll merge this in and cut a patch release shortly!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.58%. Comparing base (acbc18f) to head (645351e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
- Coverage   71.62%   71.58%   -0.04%     
==========================================
  Files          48       48              
  Lines        3027     3027              
  Branches      479      479              
==========================================
- Hits         2168     2167       -1     
- Misses        744      745       +1     
  Partials      115      115              

Flag	Coverage Δ
unittests	71.55% <ø> (-0.04%)	⬇️

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