Fixes specification of definitions directories in Python 3.9

The trick for converting a MultiplexedPath to a regular Path, using `joinpath('')` does not work in Python 3.9. It turns out that it was probably not needed in the latest version anyway.

Author: rayosborn

src/nxvalidate/validate.py

@@ -85,10 +85,9 @@ def __init__(self, definitions=None):
             elif isinstance(definitions, Path):
                 self.definitions = definitions.resolve()
             else:
-                self.definitions = definitions.joinpath('')
+                self.definitions = definitions
         else:
-            self.definitions = package_files(
-                'nxvalidate.definitions').joinpath('')
+            self.definitions = package_files('nxvalidate.definitions')
         self.baseclasses = self.definitions / 'base_classes'
         if not self.baseclasses.exists():
             raise NeXusError(f'"{self.baseclasses}" does not exist')