Python 3.7 Support

Update README.rst

Starting with Python 3.7, __aiter__ must return an asynchronous iterator object. Returning anything else will result in a TypeError error.

https://docs.python.org/3/reference/datamodel.html#asynchronous-iterators

Minor tweak in "Writing tests for aiofiles": s/threadpool.wrap.register/aiofiles.threadpool.wrap.register

Update README.rst

Update classifiers through Python 3.6

Added Manifest file

Make sendfile_file test conditional

DRY up the os module

For certain kernel versions, the test fails with OSError: [EINVAL]

According to man(2) sendfile:

In Linux 2.4 and earlier, out_fd could also refer to a regular file; this
possibility went away in the Linux 2.6.x kernel series, but was restored in
Linux 2.6.33

This patch skips the test sendfile_file on kernel versions which fall in the
above range

Test on Py 3.6