fix: add callable check to progress_bar

If `progress_func` is `None` a `TypeError` (`TypeError: 'NoneType'
object is not callable`) is raised. Add check to verify that
`progress_func` is callable.

Fixes #378.

artifactory.py

@@ -1422,7 +1422,7 @@ def writeto(self, pathobj, file, chunk_size, progress_func):
 
             file.write(chunk)
 
-        if real_chunk > 0:
+        if callable(progress_func) and real_chunk > 0:
             progress_func(bytes_read + real_chunk, file_size)