Replace which command subprocesses with shutil.which()

[bwoodsend]

This fixes (hidden by default) warnings about the subprocesses' stdout/stderr handles not being closed before being garbage collected:

> PYTHONWARNINGS=always::Warning pytest
...
../../../home/brenainn/.pyenv/versions/3.12.0/lib/python3.12/site-packages/pyscreeze/__init__.py:81
  /home/brenainn/.pyenv/versions/3.12.0/lib/python3.12/site-packages/pyscreeze/__init__.py:81: ResourceWarning: unclosed file <_io.BufferedReader name=11>
    whichProc = subprocess.Popen(['which', 'scrot'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
  Enable tracemalloc to get traceback where the object was allocated.
  See https://docs.pytest.org/en/stable/how-to/capture-warnings.html#resource-warnings for more info.

../../../home/brenainn/.pyenv/versions/3.12.0/lib/python3.12/site-packages/pyscreeze/__init__.py:81
  /home/brenainn/.pyenv/versions/3.12.0/lib/python3.12/site-packages/pyscreeze/__init__.py:81: ResourceWarning: unclosed file <_io.BufferedReader name=13>
    whichProc = subprocess.Popen(['which', 'scrot'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
  Enable tracemalloc to get traceback where the object was allocated.
  See https://docs.pytest.org/en/stable/how-to/capture-warnings.html#resource-warnings for more info.

And it removes the awkwardness of minimal Linux environments that don't have which installed.