Only disable InsecureRequestWarning when verify_cert=False.

[mblayman]

My team is upgrading to the latest Mixpanel version, and we noticed the behavior change of verify_cert moving to False by default.

We would like to use verify_cert=True, but I noticed that Mixpanel.__init__ is disabling urllib3 warnings regardless of the verify_cert value. By disabling this warning in urllib3 unguarded in __init__, I think the Mixpanel library is effectively disabling this warning for any libraries that use urllib3.

This branch changes that so that the warnings will only be disabled if the verify_cert value is left in its more insecure default mode of False.

Thanks for the consideration!

[seizethedave]

Looks good to me. Thanks for the PR!

[seizethedave]

Released: https://github.com/mixpanel/mixpanel-python/releases/tag/4.8.4

[seizethedave]

@mblayman Thanks for reminding me that things were in this state. I have just released #103 as 4.9.0 letting requests manage all of the TLS cert administrivia, and turning verification back on by default. Let me know how it works for you.

[mblayman]

I appreciate the update. My team hasn't updated yet because we're not always right on top of dependency upgrades. I'm guessing it will work great because requests is solid, but I'll report back if we hit any issues. Thanks for taking care of this library!