Fix urllib3 getheaders/getheader deprecation in RESTResponse (#195)

[CedricConday]

Fixes #195

urllib3 deprecated HTTPResponse.getheaders() and HTTPResponse.getheader() in favour of accessing HTTPResponse.headers directly, and has scheduled them for removal. RESTResponse wrapped those deprecated accessors, so every response emitted a DeprecationWarning and the SDK would break once urllib3 drops them.

Change

RESTResponse now reads headers via .headers / .headers.get(name, default):

def getheaders(self):
    return self.urllib3_response.headers

def getheader(self, name, default=None):
    return self.urllib3_response.headers.get(name, default)

This is behaviour-preserving — in urllib3 2.x getheaders() returns self.headers and getheader(name, default) returns self.headers.get(name, default). The public RESTResponse.getheaders()/getheader() methods are unchanged, so the call sites in api_client/__init__.py and exceptions/__init__.py keep working.

Tests

Added tests/test_api_client/test_rest.py with version-independent regression tests: a spy response whose deprecated accessors raise if called, plus a check against a real urllib3 HTTPResponse. The spy tests fail on the old code and pass on the fix. Full test_api_client suite (150 tests) stays green.

---

AI-assisted, human-reviewed — I'm an AI engineer; I find, fix, and test with AI, then review and verify before opening.