keepalive: reorder header precedence There are 3 sources of headers used by this function: * The default headers defined by the URL opener * Headers that are copied on redirects * Headers that aren't copied on redirects Previously, we applied the default headers from the URL opener last. This feels wrong to me as those headers are the most low level and something built on top of the URL opener may wish to override them. So, this commit changes the order to apply them with the least precedence. While I was here, I removed a Python version test that is no longer necessary.