Remove M2Crypto dependency

[aprams]

M2Crypto currently implies a heavy burden in terms of installing (see e.g. #176).

Using an alternative, more modern library might help to overcome this burden, which can not be easily solved an update in the requirements.txt.

An example would be this repository, which used the cryptography package:

https://github.com/anfema/django-ses-sns-tracker/blob/e65146fe204088f73c9240bef6cd8e416efb82f1/ses_sns_tracker/utils.py

I'm by no means an SSL expert, so I'm not sure about the comparability, but it would be a major QoL improvement.

[pcraciunoiu]

@aprams thanks for filing and linking to a suggestion! Since I'm not actively using this package, it's tricky for me to do full hands-on testing. But, if you'd like to submit a PR that you've tested, I'm happy to review and try to get it tested and merged.

I like that it was introduced using a setting, so it could be the same here perhaps - giving people the option to stick with M2Crypto or choosing cryptography?

[joshsny]

+1 for this issue. Trying to use django-ses[events] in production on Heroku but can't find a way to install m2crypto. Any ideas for how to get round this?

I'd love to submit a PR, unfortunately I do not have the expertise :(

[joshsny]

For anyone looking at this who isn't bothered about verifying the events but just wants to receive them, setting AWS_SES_VERIFY_EVENT_SIGNATURES = False (not VERIFY_EVENT_SIGNATURES = False) did the trick.

Edit: I've submitted a PR to update the README with the correct flag.

[pcraciunoiu]

#206 and it was merged. Thanks!