apple developer token取得作業時にimport errorが発生してしまいます。どなたかアドバイスをお願いします！

apple developer tokenの取得

ここに質問の内容を詳しく書いてください。
（例）PHP(CakePHP)で●●なシステムを作っています。

jwtを使ってtoken取得時、編集したmusic_token.pyを実行した際に以下のエラーメッセージが発生しました。

発生している問題・エラーメッセージ

Traceback (most recent call last):
  File "music_token.py", line 34, in <module>
    token = jwt.encode(payload, secret, algorithm=alg, headers=headers)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/jwt/api_jwt.py", line 56, in encode
    json_payload, key, algorithm, headers, json_encoder
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/jwt/api_jws.py", line 102, in encode
    key = alg_obj.prepare_key(key)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/jwt/algorithms.py", line 349, in prepare_key
    key = load_pem_public_key(key, backend=default_backend())
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/cryptography/hazmat/backends/__init__.py", line 15, in default_backend
    from cryptography.hazmat.backends.openssl.backend import backend
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/cryptography/hazmat/backends/openssl/__init__.py", line 7, in <module>
    from cryptography.hazmat.backends.openssl.backend import backend
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/cryptography/hazmat/backends/openssl/backend.py", line 16, in <module>
    from cryptography import utils, x509
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/cryptography/x509/__init__.py", line 8, in <module>
    from cryptography.x509.base import (
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/cryptography/x509/base.py", line 16, in <module>
    from cryptography.x509.extensions import Extension, ExtensionType
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/cryptography/x509/extensions.py", line 18, in <module>
    from cryptography.hazmat.primitives import constant_time, serialization
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/cryptography/hazmat/primitives/constant_time.py", line 9, in <module>
    from cryptography.hazmat.bindings._constant_time import lib
ImportError: dynamic module does not define init function (init_constant_time)