Pin Flask to <2.0 to fix the test suite

httpbin does not limit Flask version and the 2.0 release has broken compatibility with our test suite

Author: Ousret

requirements-dev.txt

@@ -3,4 +3,5 @@ pytest-cov
 pytest-httpbin<1.0
 pytest-mock==2.0.0
 httpbin==0.7.0
+Flask>=1.0,<2.0
 wheel