Content-length and chunk size parsing now strictly matches the RFCs.
We previously used the python int() function which accepted leading
plus signs and internal underscores, which are not allowed by the
HTTP RFCs (it also accepts minus signs, but these are less problematic
in this context since they'd result in errors elsewhere)

It is important to fix this because when combined with certain proxies,
the lax parsing could result in a request smuggling vulnerability (if
both Tornado and the proxy accepted an invalid content-length but
interpreted it differently). This is known to occur with old versions
of haproxy, although the current version of haproxy is unaffected.

Fix case-sensitivity for Content-Encoding: gzip. as per rfc7231 the Content-Encoding values are case-insensitive https://datatracker.ietf.org/doc/html/rfc7231#section-3.1.2.1

* maintainance -> maintenance

* recieving -> receiving

"Implicit-optional" mode is on by default, but that default is intended to change in the indefinite future (python/peps#689, python/typing#275). Go ahead and change to the future explicit use of Optional.

The "provisional" typing module in 3.5.2 is kind of broken/incomplete
so we need to use more forward references to avoid confusing it. The
significance of this version in particular is that it was the one
included in ubuntu 16.04.

Fixes #2604

After a redirect, the chunked-encoding header is already set and would
not be detected correctly. This affects empty bodies with
allow_nonstandard_methods and any use of body_producer.

It's difficult to synchronize test shutdown with the exits of all
coroutines, so explicitly cancel all native coroutines (which are
spammy when allowed to be GC'd).

Suppress logging of CancelledErrors in IOLoop.

The PR #2449 was incomplete, it omited consideration of another
branch of the 'flush' method. In short, the future returned by
'RequestHandler.finish()' may also be the return value of
'HTTP1Connection.write()'. So we should make the same change to
'HTTP1Connection.write()'.

Fix #2448.

`black` refused to move the operator across the comment, leaving it
stranded on a line by its own. The most recent version of pycodestyle
complains about this.

It occasionally makes some odd-looking decisions and uses a lot of
vertical space but overall it's a big improvement, especially for the
dense type signatures.

Also deprecates callback arguments in httputil and http1connection

This method is not actually redundant with
HTTPMessageDelegate.on_connection_close (maybe there is a useful
refactoring that would make it so, but for now it's still useful)

Fixes #2280

setup.py is intended to be run with "python setup.py install"

sources in the tornado module which have main functions/clauses
are intended to be run with "python -m tornado.xxx"

various demos and scripts can be run directly

bug introduced in #2225

otherwise a subsequent request on the same connection
will fail to be parsed

thanks to @eeelin for the bug report

E126 continuation line over-indented for hanging indent
E128 continuation line under-indented for visual indent
E129 visually indented line with same indent as next logical line

bug introduced in #2225

otherwise a subsequent request on the same connection
will fail to be parsed

thanks to @eeelin for the bug report