Replace `isinstance(self.content, file)` with a duck type for the `read()` method. Having a read method is what PEP 333 defines as the minimum requirement for a "file-like" object.

This fixes a deprecation warning on 3.7.

httpservers writes an empty string or an internal server
error message, these needs to be bytes in python 3.

It might have been useful to have wsgi_write_chunk accept
either bytes or strings and do the right thing, but that
seemed too invasive to be safe.

This reverts commit 78dd2ec0138467305f1686558fca4ff8ca0b2b70.

That removes the use of the so-called "thread safe" tsafe module, which
adds a lock around many messages, but did not account for the use of
that lock in the paste/httpserver.py module.

An attempt was made to just get rid of that lock, but since there is
limited testing of that area, and few resources to confirm the change,
it's been decided that keeping tsafe and allow the deprecation warning
to be exposed is the best thing to do. With luck the decpration warning
will encourage people to not use the httpserver and choose something
else instead.

Fixes #19

It would fail if the length of bytes and unicode differs.

It now passes non-bytes to `Form` directly.

* py3 fixes for form handling in paste.fixture

It uses "not six.PY2" in contrast to other places in the code to be
forward-compatible.

I've not looked too closely, but it might make sense to decode body/text
in the beginning already, instead of having it as bytes internally?!

Also, like mentioned in [1] already, it should probably use the correct
source encoding?!

1: https://github.com/cdent/paste/blob/36e5b8bd16a6063ec654faf04541f3a20d19f7fe/paste/fixture.py#L820

* Add test, using/fixing SlowConsumer form app

* pytest: fix collection warnings via __test__=False

Fixes

> "cannot collect test class %r because it has a __init__ constructor

Ref: https://github.com/pytest-dev/pytest/issues/2007

* pytest: configure testpaths

This is faster with test collection.

* pytest: fix warning with doctests

Fixes

> /usr/lib/python3.7/site-packages/_pytest/python.py:764:
> RemovedInPytest4Warning: usage of Generator.Function is deprecated,
> please use pytest.Function instead

* Minor fixes around s/py.test/pytest/