Bump pyjwt and cryptography versions (#749)

* Bump pyjwt and cryptography versions

Bump PyJWT to v2. No backward incompatible changes in the new version
affects oauthlib.

Bump minimum cryptography version to 3. PyJWT bumps cryptography as
well plus older versions don't support newer versions of openssl.

* Remove python 3.5 support; add python 3.8 support

- Python 3.5 has reached end-of-life.
- Add support for Python 3.8
- Move dist to bionic (xenial is EOL in April)
- Upgrade pip in travis.yml's `before_install`. this should install a wheel
  for pyca/cryptography
- Install rust for pypy builds. It is a requirement for cryptography as no
  pypy wheels exist.

Co-authored-by: Asif Saif Uddin <auvipy@gmail.com>

requirements.txt

@@ -1,3 +1,3 @@
-pyjwt==1.7.1
+pyjwt>=2.0.0,<3
 blinker==1.4
-cryptography>=1.4.0
+cryptography>=3.0.0,<4

setup.py

@@ -16,8 +16,8 @@ def fread(fn):
         return f.read()
 
 
-rsa_require = ['cryptography>=1.4.0']
-signedtoken_require = ['cryptography>=1.4.0', 'pyjwt>=1.6.0']
+rsa_require = ['cryptography>=3.0.0,<4']
+signedtoken_require = ['cryptography>=3.0.0,<4', 'pyjwt>=2.0.0,<3']
 signals_require = ['blinker>=1.4.0']
 
 setup(