Fix bout-add-mod-path for Python 3.9

[dschwoerer]

I got this error - so might be a change in python 3.9

Traceback (most recent call last):
  File "/home/dave/soft/BOUT-dev/bin/bout-add-mod-path", line 147, in <module>
    create_mod(modulepath, name, top)
  File "/home/dave/soft/BOUT-dev/bin/bout-add-mod-path", line 79, in create_mod
    pathlib.Path.mkdir(modulepath+"/bout++", parents=True, exist_ok=True)
  File "/usr/lib64/python3.9/pathlib.py", line 1312, in mkdir
    self._accessor.mkdir(self, mode)
AttributeError: 'str' object has no attribute '_accessor'