BLD: fix build failure py3.9.1 on OSX
Ran `brew upgrade` yesterday and ended up bumping from 3.9.0 to 3.9.1, after which I got build failures 

```
    LooseVersion(python_target) < "10.9"
  File "/usr/local/Cellar/python@3.9/3.9.1_2/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/version.py", line 306, in __init__
    self.parse(vstring)
  File "/usr/local/Cellar/python@3.9/3.9.1_2/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/version.py", line 314, in parse
    components = [x for x in self.component_re.split(vstring)
TypeError: expected string or bytes-like object
```

No idea why this just started breaking now.

