unpin cryptography

we previously pinned cryptography to versions less than 3.4 since after
that point, cryptography started using rust as a dependency. This isn't
an issue if you install from wheel, but we found that the version of pip
bundled with the python buildpack was too old to support this. However,
since upgrading from python 3.6 to python 3.9, the pip version has been
bumped and we now no longer need to pin cryptography as it installs
correctly.

requirements.in

@@ -26,10 +26,6 @@ beautifulsoup4==4.9.3
 lxml==4.7.1
 Werkzeug==2.0.2
 
-# higher version causes build to fail on PaaS due to lack of Rust
-# see https://github.com/pyca/cryptography/issues/5810
-cryptography<3.4 # pyup: <3.4
-
 notifications-python-client==6.0.2
 
 # PaaS

requirements.txt

@@ -47,7 +47,6 @@ cffi==1.14.5
     # via
     #   -r requirements.in
     #   bcrypt
-    #   cryptography
 charset-normalizer==2.0.7
     # via requests
 click==8.0.3
@@ -68,8 +67,6 @@ click-repl==0.2.0
     # via celery
 colorama==0.4.3
     # via awscli
-cryptography==3.3.2
-    # via -r requirements.in
 dnspython==1.16.0
     # via eventlet
 docopt==0.6.2
@@ -221,7 +218,6 @@ six==1.16.0
     #   bcrypt
     #   bleach
     #   click-repl
-    #   cryptography
     #   eventlet
     #   flask-marshmallow
     #   jsonschema