add support for update_into on CipherContext (#3190)

* add support for update_into on CipherContext

This allows you to provide your own buffer (like recv_into) to improve
performance when repeatedly calling encrypt/decrypt on large payloads.

* another skip_if

* more skip_if complexity

* maybe do this right

* correct number of args

* coverage for the coverage gods

* add a cffi minimum test tox target and travis builder

This tests against macOS so we capture some commoncrypto branches

* extra arg

* need to actually install py35

* fix

* coverage for GCM decrypt in CC

* no longer relevant

* 1.8 now

* pep8

* dramatically simplify

* update docs

* remove unneeded test

* changelog entry

* test improvements

* coverage fix

* add some comments to example

* move the comments to their own line

* fix and move comment
diff --git a/setup.py b/setup.py
index 1b1ff60..1ec3a35 100644
--- a/setup.py
+++ b/setup.py
@@ -36,6 +36,7 @@
 requirements = [
     "idna>=2.0",
     "asn1crypto>=0.21.0",
+    "packaging",
     "six>=1.4.1",
     "setuptools>=11.3",
 ]