certifi dropped Python 2 support. (#356)

tests/utils/constraints.txt

@@ -1,4 +1,5 @@
 bcrypt < 3.2.0 ; python_version <= '3.6'
+certifi < 2022.5.18 ; python_version < '3.5' # certifi 2022.5.18 requires Python 3.5 or later
 cffi >= 1.14.2, != 1.14.3 # Yanked version which older versions of pip will still install
 coverage >= 4.2, < 5.0.0, != 4.3.2 ; python_version <= '3.7' # features in 4.2+ required, avoid known bug in 4.3.2 on python 2.6, coverage 5.0+ incompatible
 coverage >= 4.5.4, < 5.0.0 ; python_version > '3.7' # coverage had a bug in < 4.5.4 that would cause unit tests to hang in Python 3.8, coverage 5.0+ incompatible