Signed webhook verification fails

<!--

If this is a feature request, make sure you search Issues for an existing request before creating a new one!



Please utilize the template below to help us resolve your issue.



Note that many issues can be resolved by updating to the latest version.

-->



### Issue Summary

`EventWebhook.convert_public_key_to_ecdsa` doesn't seem to be compatible with the version 2.0.0 of [starkbank-ecdsa](https://github.com/starkbank/ecdsa-python) released a couple of days ago. Installing the older starkbank-ecdsa==1.0.0 fixes the issue.



### Steps to Reproduce

Call `convert_public_key_to_ecdsa` with a valid public key

or

Run tests in `test/test_eventwebhook.py`



### Code Snippet

```python

from sendgrid.helpers.eventwebhook import EventWebhook

PUBLIC_KEY = 'MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE83T4O/n84iotIvIW4mdBgQ/7dAfSmpqIM8kF9mN1flpVKS3GRqe62gw+2fNNRaINXvVpiglSI8eNEc6wEA3F+g=='

ew = EventWebhook()

ew.convert_public_key_to_ecdsa(PUBLIC_KEY)

```



### Exception/Log

```

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "/Users/xxx/sendgrid-python/sendgrid/helpers/eventwebhook/__init__.py", line 30, in convert_public_key_to_ecdsa

    return PublicKey.fromPem(public_key)

  File "/usr/local/lib/python3.9/site-packages/ellipticcurve/publicKey.py", line 39, in fromPem

    publicKeyPem = getPemContent(pem=string, template=_pemTemplate)

  File "/usr/local/lib/python3.9/site-packages/ellipticcurve/utils/pem.py", line 6, in getPemContent

    return search("".join(pattern.splitlines()), "".join(pem.splitlines())).group(1)

AttributeError: 'NoneType' object has no attribute 'group'

```



### Technical details:

Tried with

* sendgrid-python version: 6.8.2

* python version: 3.9.5



and

* sendgrid-python version: 6.7.1

* python version: 3.6.13

[]