Python 3.6 compatibility: random bits & bcrypt

Switched from Sybren's RSA library to the new stdlib module 'secrets' to
generate secret tokens. This also means that the rsa library was demoted
to secondary requirement.

requirements.txt

@@ -3,7 +3,7 @@
 
 attrs==16.2.0
 algoliasearch==1.8.0
-bcrypt==2.0.0
+bcrypt==3.1.3
 blinker==1.4
 bugsnag==2.3.1
 bleach==1.4.3
@@ -27,7 +27,6 @@ pycrypto==2.6.1
 pyOpenSSL==0.15.1
 python-dateutil==2.5.3
 requests==2.9.1
-rsa==3.4.2
 simplejson==3.8.2
 WebOb==1.5.0
 wheel==0.29.0
@@ -52,5 +51,6 @@ pyasn1-modules==0.0.8
 pymongo==3.2.2
 pyparsing==2.1.10
 requests-oauthlib==0.7.0
+rsa==3.4.2
 six==1.10.0
 WTForms==2.1