Make theme dir writable after copying to it

In certain settings (i.e. when using the nix package manager), program files are
stored as read-only. Hence, copying the themes from there will result in
read-only directories in the output directory, making subsequent operations fail
due to permission errors.

There does not seem to be a way to let `shutil.copytree` "forget" permissions,
so we adapt them after the fact.

sigal/writer.py

@@ -24,6 +24,7 @@ import importlib
 import logging
 import os
 import shutil
+import stat
 import sys
 import types
 
@@ -105,6 +106,15 @@ class AbstractWriter:
         ):
             shutil.copytree(static_path, self.theme_path, dirs_exist_ok=True)
 
+            # Ensure that the theme dir is writeable
+            for root, _, files in os.walk(self.theme_path):
+                st = os.stat(root)
+                os.chmod(root, st.st_mode | stat.S_IWUSR)
+                for name in files:
+                    path = os.path.join(root, name)
+                    st = os.stat(path)
+                    os.chmod(path, st.st_mode | stat.S_IWUSR)
+
         if self.settings["user_css"]:
             if not os.path.exists(self.settings["user_css"]):
                 self.logger.error(