bump the minimum cryptography version (#875)

Users with older cryptography (and hence potentially older asn1crypto, a
transitive dependency) are seeing a serious bug on macOS catalina due to
the way older asn1crypto loads a shared library. While this isn't a
pyOpenSSL bug bumping this dep might prevent the bug from impacting
some users.
diff --git a/setup.py b/setup.py
index f6d5499..3e76199 100755
--- a/setup.py
+++ b/setup.py
@@ -94,7 +94,7 @@
         package_dir={"": "src"},
         install_requires=[
             # Fix cryptographyMinimum in tox.ini when changing this!
-            "cryptography>=2.3",
+            "cryptography>=2.8",
             "six>=1.5.2"
         ],
         extras_require={