Add python_requires>=3.5

We no longer support python2, but this is the only way i've found to say
that the package can only be installed on python 3. 3.5 is somewhat of
an arbitrary value, but I don't want to go perfectly recent.

setup.cfg

@@ -28,6 +28,7 @@ classifiers =
 [options]
 packages = requests_mock
 include_package_data = true
+python_requires = >=3.5
 install_requires =
  requests>=2.22,<3