Simplify return type for fix_script

We always pass a file path to this function, so assert as much. We want
the return type to be consistent so we can assign the result to
non-Optional types.

src/pip/_internal/operations/install/wheel.py

@@ -105,13 +105,12 @@ def csv_io_kwargs(mode):
 
 
 def fix_script(path):
-    # type: (text_type) -> Optional[bool]
+    # type: (text_type) -> bool
     """Replace #!python with #!/path/to/python
     Return True if file was changed.
     """
     # XXX RECORD hashes will need to be updated
-    if not os.path.isfile(path):
-        return None
+    assert os.path.isfile(path)
 
     with open(path, 'rb') as script:
         firstline = script.readline()