Add PyJwT version 2.0 compatability and update required PyJwT to v2.0

This was the change that broke our usage.
```
Dropped deprecated require_* options in jwt.decode(...)
For example, instead of jwt.decode(encoded, key, algorithms=["HS256"], options={"require_exp": True}), use jwt.decode(encoded, key, algorithms=["HS256"], options={"require": ["exp"]}).
```
I switched to the new require syntax and everything started working again. Unfortunately the new fix doesn't work on old versions so I also updated the requirements.txt.

duo_universal/client.py

@@ -298,9 +298,10 @@ def exchange_authorization_code_for_2fa_result(self, duoCode, username, nonce=No
                 issuer=OAUTH_V1_TOKEN_ENDPOINT.format(self._api_host),
                 leeway=LEEWAY,
                 algorithms=["HS512"],
-                options={'require_exp': True,
-                         'require_iat': True,
-                         'verify_iat': True},
+                options={
+                    'require': ['exp', 'iat'],
+                    'verify_iat': True
+                },
             )
         except Exception as e:
             raise DuoException(e)

requirements.txt

@@ -1,5 +1,5 @@
 cryptography>=3.2
-PyJWT>=1.7.1, < 2
+PyJWT>=2.0
 pyOpenSSL>=19.0.0
 requests>=2.22.0
 wheel>=0.35.1