[deps] force flask<=1.0.0 (#4959)

flask 1.0 came out and has backwards incompatible changes. People
are reporting that fresh install doesn't work anymore.

fixes #4953

We should ship a 0.25.1 with this in

(cherry picked from commit 2c5200a)

setup.py

@@ -55,7 +55,7 @@ def get_git_sha():
         'celery',
         'colorama',
         'cryptography',
-        'flask',
+        'flask<=1.0.0',
         'flask-appbuilder',
         'flask-cache',
         'flask-compress',