fix: HTTPResponse object has no attribute 'chunked' (#137)

[princ0301]

Fixes #137

Summary

The chunked attribute on urllib3.response.HTTPResponse is not present in older versions of the library, which can be used by older versions of requests. This change uses getattr to safely access the attribute, preventing an AttributeError.

Validation

Tests passing after 1 attempt(s).

tests/test_expires_heuristics.py ................                        [ 59%]
tests/test_max_age.py ..                                                 [ 61%]
tests/test_redirects.py ....                                             [ 65%]
tests/test_regressions.py ..                                             [ 67%]
tests/test_serialization.py ..........                                   [ 77%]
tests/test_server_http_version.py .                                      [ 78%]
tests/test_storage_filecache.py ..................                       [ 96%]
tests/test_storage_redis.py ...                                          [ 99%]
tests/test_vary.py .                                                     [100%]
============================= 101 passed in 3.18s ==============================