The concurrency test is essentially a race between a file read,
with a 1 second delay, and 40 1-byte net reads, with a 10msec delay.

Pypy can fail this test, for reasons that appear to have nothing to
do with the goal of the test.

Ease the goal to allow pypy to catch up.

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