DeprecationWarning: HTTPResponse.getheader() is deprecated and will be removed in urllib3 v2.1.0.

[snijbell]

Getting the following warning:

DeprecationWarning: HTTPResponse.getheader() is deprecated and will be removed in urllib3 v2.1.0. Instead use HTTPResponse.headers.get(name, default). return self.urllib3_response.getheader(name, default)

A previous issue #71 covered the case of all headers, but the single header case also needs a change.

https://github.com/muxinc/mux-python/blob/70431a246e55d2db80e5f0a701f7fb0f737d6554/mux_python/rest.py#L47