v6.0.0 Replace Requests with HTTPX

[Deconstrained]

Justification

Long-term, we are looking to add support for thread safety, i.e. to fulfill #132. This requires switching the HTTP client on which pdpyras is based to one that is thread-safe. HTTPX is thread-safe and provides an interface that is very similar to that of Requests, and so minimal changes will be necessary in order to adopt it. Moreover, it optionally supports HTTP/2 which gives us the option of supporting that protocol in the future as well. Finally, allegations regarding how the maintainers of Requests have responded to responsible security disclosures give us pause, and compel us to switch out the base HTTP client for the benefit of PagerDuty customers' security.

Summary of Changes

• Replace requests.Session with httpx.Client and requests.Response with httpx.Response
• Deprecate the "prepare_headers" method, which was not needed and complicated integration with HTTPX
• Add a new "default_headers" property to take its place

To-do

• Validate that sending GET requests with content-type: application/json won't mess with how query parameters are interpreted and thus that it is safe to proceed with the simplification that pays off a bit of tech debt
• Enumerate all the possible exception classes that could be raised by httpx and its dependencies and update the error handling logic to recognize them
• Write down the full list of deprecations and breaking changes (if any)
• Test the various functions of the new client using a live key
• Add a basic integration test to CircleCI (could be any single Python version, it needn't be all of them) using the same API key as publicly given for demo purposes in the developer documentation

[Deconstrained]

Note on test failures: the second to-do item above should take care of them, because the library currently makes the assumption that the local environment has urllib3 by virtue of installing requests (it's a second-removed upstream dependency).