* Test on OpenSSL 1.1.0 w/ Debian stretch

* Make pyOpenSSL compatible with openssl 1.1.0 again

Co-authored-by: Shane Harvey <shnhrv@gmail.com>

* remove py2 w/ cryptography master

* Update ci.yml

* Tox.ini; Test on Python 3.9 and make flake8 stricter

* max-line-length = 88

* Remove unused import

* Update tox.ini

* Use PEP8 line length

* Use PEP8 line length

* fix a memleak

* black

* Keep reference to SSL verify_call in Connection object

If a set_verify is used on a context before and after a Connection
the reference in the SSL* object still points to the old _verify_helper
object. Since this object has no longer any references to it, the
callback can result in a segfault.

This commit fixes the issues by ensuring that as long as the
Connection object/SSL* object lives a reference to the callback
function is held.

* Add Unit test for set_verify_callback deference

It's not being run anywhere, and plainly doesn't support py3 so it's clearly not important to anyone.