Improve user-agent ordering

[jaraco]

According to RFC 7231, "the product identifiers are listed in decreasing order of their significance for identifying user agent software."

Setuptools instead supplies a user agent like "Python-urllib/3.5 setuptools/21.2.2".

Better would be for setuptools to follow the RFC, placing 'setuptools' first.

[jaraco]

Supplying setuptools first will also be more consistent with the expectations for other package installers and avoid issues like devpi 354.