blocksize is hardcoded to 8192 in send() and _read_readable(), preventing
efficient upload when using file-like body.

Users of the module that are not interested in chunked encoding can rewrite
the copy loop using HTTPConnection.send():

   conn = HTTPSConnection(...)
   conn.putrequest(...)
   conn.putheader(...)
   conn.endheaders()

   while True:
       chunk = file.read(512*1024)
       if not chunk:
          break
       conn.send(chunk)

But fixing send() to use a configurable blocksize seems more useful.

Also, users of requests do not have access the underlying connection, so
they cannot use preferred buffer size.

When reading from /dev/zero and uploading to server that drop the received
data, larger buffer size gives 3X more throughput *and* 1/3 of cpu time.
With real storage and network, the effect will probably be much smaller.

When using highlevel request() api, users can control the block size by
wrapping the file object with an iterator:

    class FileIter:

        def __init__(self, file, blocksize):
            self.file = file
            self.blocksize = blocksize

        def __iter__(self):
            while True:
                datablock = self.file.read(self.blocksize)
                if not datablock:
                    break
                yield datablock

Adding configurable block size will avoid this workaround.

New changeset ad455cd9243319b896c86074ffeb3bf78a82f4ec by Victor Stinner (Nir Soffer) in branch 'master':
bpo-31945: Configurable blocksize in HTTP(S)Connection (#4279)
https://github.com/python/cpython/commit/ad455cd9243319b896c86074ffeb3bf78a82f4ec

Thank you Nir Soffer for this nice enhancement. Sadly, since it's a new feature, it cannot be backport to Python 3.6 nor 2.7, since it's a new feature.

The commit message contains much more information than the NEWS and What's New entries. Maybe the What's New entry can be completed, I don't know. In the meanwhile, I close the issue :-) Nir: Feel free to create a new PR if you want to complete the What's New entry ;-)