flask-restx 0.3.0 doesn't work with flask 2.0 (#52)

Looks like we hit flask's release spot: flask 2.0 was released earlier
today two hours ago, which are within the window in which we released a
new MAX Base version after test. Fix flask < 2.0 for now.

Author: xuhdev

requirements.txt

@@ -1,3 +1,5 @@
 flask-cors==3.0.9
 flask-restx==0.3.0
+# The following line is a hack: flask-restx doesn't work with flask 2.0. Once flask-restx fixed the issue, the following line should be removed
+flask>=1.1.2,<2.0
 maxfw==1.1.6