Add User-Agent header when resolving via urllib (#663)
Add User-Agent header when resolving via urllib (#663)

At least some Python Index servers return a simple format of packages page, redirecting to / to /+simple/.

So there is a different result between `pex` run using `urllib` and using `requests` - in one case it errors.
jsirois/pex,jsirois/pex,pantsbuild/pex,pantsbuild/pex