Ensure Compatibility with flask_frozen v1.0.1 (#182)

This commit addresses an AttributeError encountered when using flask_frozen v1.0.1 with our CustomFreezer class. Previously, the CustomFreezer's root property was returning a string representation of
the cf_output_dir, which caused compatibility issues with the latest version of flask_frozen.

This update resolves the AttributeError ('str' object has no attribute 'mkdir') and ensures our CustomFreezer class works seamlessly with flask_frozen v1.0.1.

requirements.txt

@@ -3,4 +3,4 @@ Werkzeug<3.0.0
 Flask<3.0.0
 markdown2
 emoji>=2.0.0,<3.0
-frozen-flask
+frozen-flask==1.0.1

slackviewer/freezer.py

@@ -1,10 +1,16 @@
 from flask_frozen import Freezer
+from pathlib import Path
 
 class CustomFreezer(Freezer):
 
     cf_output_dir = None
 
     @property
     def root(self):
-        return u"{}".format(self.cf_output_dir)
-
+        # Use the specified cf_output_dir if set
+        if self.cf_output_dir:
+            return Path(self.cf_output_dir)
+        # Otherwise, follow the default behavior of flask_frozen
+        else:
+            root = Path(self.app.root_path)
+            return root / self.app.config['FREEZER_DESTINATION']