Add cryptography to dependencies

Required by requests-http-signatures. Which is a shame since
we already use pycryptodome.

setup.py

@@ -28,6 +28,7 @@ setup(
     license="BSD 3-clause",
     install_requires=[
         "attrs",
+        "cryptography",
         "cssselect>=0.9.2",
         "dirty-validators>=0.3.0",
         "lxml>=3.4.0",