bump requests requirement to >=2.10 (#4248)

acme/setup.py

@@ -15,7 +15,11 @@ install_requires = [
     'PyOpenSSL>=0.13',
     'pyrfc3339',
     'pytz',
-    'requests[security]>=2.4.1',  # security extras added in 2.4.1
+    # requests>=2.10 is required to fix
+    # https://github.com/shazow/urllib3/issues/556. This requirement can be
+    # relaxed to 'requests[security]>=2.4.1', however, less useful errors
+    # will be raised for some network/SSL errors.
+    'requests[security]>=2.10',
     # For pkg_resources. >=1.0 so pip resolves it to a version cryptography
     # will tolerate; see #2599:
     'setuptools>=1.0',