Rename all 'async' to 'async_' for compatibility with Python 3.7.

Travis is using 3.4.6. I can't reproduce the failures locally with 3.4.4 or 3.6.4.

Fixes #113

Fixes #104.

I don't think any of those attributes were part of ZEO's public API.

Disallow using strings as addresses on Windows. Fixes #106.

No specific test case, though.

Move non-test parts of ZEO.tests.forker to ZEO._forker for ZEO.server.

FIxes #105.

Add zodbpickle to server_find_global.

Some of the tests are being killed because we don't produce any output
when running parallel tests in a layer with -v1.

The amount of warnings made it hard to see real failures.

Also use more recent PyPy, and test on Py 3.6 (which produces the
'async' warnings we'll use to fix #104)

Return early after decode error.

Closes #107.

Add coverage for ZEO module convenience functions.

Closes #102.

Closes #102.

fix ZopeUndo.Prefix decode on python 2

Use python_requires and extras_require for version-specific deps

Fixes #93.
Fixes #92

Example:

```
$ pip install .
Processing /.../ZEO
ZEO requires Python '>=2.7.9,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*' but the running Python is 3.3.6`
```

runzeo clears socket files no matter what type of string address is

We need to use six.string_types in a few other places to account for the change in ZConfig. unix_domain_sockets() was failing before this on Python 2.

They now expire in the year 4755.

Fixes #90

Also remove an unused and duplicate import from that file and the
redundant `test_suite` declaration from the test file. There was also
a duplicate method definition in the test file that was removed.