Find schemas using Path module and avoid UNC paths on Windows

[geographika]

Fix for #207.

@peterschaer - if you are able to test and confirm this fix, that would be great. A new mappyfile release is on its way!

On Windows using absolute should keep the driver letter, and not use the UNC name:

base = Path("K:/temp").parent
WindowsPath('K:/')
>>> str((base / "schemas").absolute())
'K:\\schemas'
>>> str((base / "schemas").resolve())
'\\\\fs.example.com\\k\\schemas'

[coveralls]

coverage: 91.52% (-0.04%) from 91.563%
when pulling b2e0de7 on unc-paths
into deabd30 on master.