Fix qmk doctor 'bytes-like object is required' on linux

This fixes the following issue related to encoding on linux systems. Add
`universal_newlines=True` to subprocess.

<class 'TypeError'>
☒ a bytes-like object is required, not 'str'
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/milc.py", line 564, in __call__
    return self.__call__()
  File "/usr/local/lib/python3.7/site-packages/milc.py", line 569, in __call__
    return self._entrypoint(self)
  File "$HOME/qmk_firmware/lib/python/qmk/cli/doctor.py", line 56, in doctor
    for line in mm_check.stdout.split('\n'):
TypeError: a bytes-like object is required, not 'str'

0 files changed, 0 insertions, 0 deletions