Flask currently supports importing app through a combination of module
path and app variable name, such as '/usr/app.py:my_app'. When the
module path contains a colon, it will conflict with this import way and
a `flask.cli.NoAppException` will be raised.
A file path on a Windows system may contain a colon followed by a slash.
So we solved this problem on Windows by ignoring the colon followed by a
slash when we split app_import_path.
Fix issue #2961.
c38499bbf2