Release 6.12.1 breaks backwards compatibility for importing EventWebhookHeader

[chad-mazlo]

I have code that follows the standard example for verifying webhooks as defined here: https://github.com/sendgrid/sendgrid-python/blob/main/examples/helpers/eventwebhook/eventwebhook_example.py

This commit breaks the example by removing the import of EventWebhookHeader from sendgrid/helpers/eventwebhook/init.py:
6eb4375#diff-5221880bcec474531b94d5979f79562574ae823a126a94aeec204806ca3f8061L5

I was able to work around this, but a patch release shouldn't break backwards compatibility like this. Also the example is now broken.

[tiwarishubham635]

Hi! I'm looking into this and will fix this at the earliest. Thanks!