Replace urllib2 HTTP requests with urllib3
dulwich.client.default_urllib2_opener has been replaced by
dulwich.client.default_urllib3_manager with otherwise similar input
arguments, returning a urllib3.PoolManager or urllib3.ProxyManager,
depending on whether a proxy configuration is given.
The opener argument to dulwich.client.HttpGitClient that took a
urllib2 opener instance has been replaced by a pool_manager argument
that takes a urllib3 pool manager instance.
Initially tested with urllib3 version 1.21, but anything later in the
1.x series should work.
Added a requirements.txt as a common convention for local setup with a
known working version. Pinned it to the latest available urllib3
version at the time.