update urllib3_response.headers.get as getheader is deprecated

[tdonia]

Problem

Follow up to #195, which updated to use urllib.headers -- there's also a deprecation for getheader coming: urllib3/urllib3#2814

Solution

Updated the call in rest.py to use .get instead of .getheader; based on the code this should be compatible with 1.2.1, our current min version

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

REST requests should work without a notice in URLLib3 2.0.4+

[jhamon]

Thanks for the attention to detail on this 🚢