Remove SDK-provided CA cert pinning

[maxbelanger]

Removes the hardcoded CA bundle from the SDK. The ca_certs parameter is still present, so users can pin with their own CA bundle if they so choose. Improves integration tests to cover both pinning using a manually-provided bundle and via the requests-provided verification mechanism (which uses certifi and/or system certificates, depending on the configuration).

It would be preferable to land #490 prior to this to enable CI.

Checklist

General Contributing

• [X] Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• [X] SDK Code Change
• [x] Example/Test Code Change

Validation

• [X] Does tox pass?
• [X] Do the tests pass?