Support copying files to a directory

Since commit dbe0b11 the `copy()` function no longer supports copying
files to a directory. The change in behaviour has lead to at least one
regression[1]. This restore the previous behaviour.

[1] #3442

Author: jerrykan

pelican/utils.py

@@ -366,6 +366,9 @@ def walk_error(err):
 
 def copy_file(source: str, destination: str) -> None:
     """Copy a file"""
+    if os.path.isdir(destination):
+        destination = os.path.join(destination, os.path.basename(source))
+
     try:
         shutil.copyfile(source, destination)
     except OSError as e: