6.4.0 version throws exceptions in Azure Functions

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### Issue Summary

Our code which was working with the last version of SendGrid throws exception about "No module named 'ellipticcurve' when Azure Python Function is invoked.



### Steps to Reproduce

1. Deploy to Azure using requirements.txt (either sendgrid or sendgrid==6.4.0)

2. Invoke Azure Function that imports SendGridAPIClient 

3. Exception is thrown: "Result: Failure Exception: ModuleNotFoundError: No module named 'ellipticcurve'."



### Code Snippet

from sendgrid import SendGridAPIClient

from sendgrid.helpers.mail import Mail

import logging

import os

...

...

...



### Exception/Log

```

Result: Failure Exception: ModuleNotFoundError: No module named 'ellipticcurve'. 



Troubleshooting Guide: https://aka.ms/functions-modulenotfound

Stack: 

File "/azure-functions-host/workers/python/3.7/LINUX/X64/azure_functions_worker/dispatcher.py", line 259, in _handle__function_load_request func_request.metadata.entry_point)

File "/azure-functions-host/workers/python/3.7/LINUX/X64/azure_functions_worker/utils/wrappers.py", line 31, in call raise extend_exception_message(e, message)

File "/azure-functions-host/workers/python/3.7/LINUX/X64/azure_functions_worker/utils/wrappers.py", line 29, in call return func(*args, **kwargs)

File "/azure-functions-host/workers/python/3.7/LINUX/X64/azure_functions_worker/loader.py", line 73, in load_function mod = importlib.import_module(fullmodname) 

File "/usr/local/lib/python3.7/importlib/__init__.py", line 127, in import_module return _bootstrap._gcd_import(name[level:], package, level)

File "/home/site/wwwroot/MyTesting/__init__.py", line 19, in <module> from .sendgrid import sendemail

File "/home/site/wwwroot/MyTesting/sendgrid/sendemail.py", line 1, in <module> from sendgrid import SendGridAPIClient

File "/home/site/wwwroot/.python_packages/lib/site-packages/sendgrid/__init__.py", line 21, in <module> from .helpers.eventwebhook import * # noqa

File "/home/site/wwwroot/.python_packages/lib/site-packages/sendgrid/helpers/eventwebhook/__init__.py", line 1, in <module> from ellipticcurve.ecdsa import Ecdsa



```



### Technical details:

* sendgrid-python version: 6.4.0

* python version: 3.7

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