* add Context.set_*_proto_version, fix #860

* docs: add new openssl tls methods

* accept the fact that nothing can be taken for granted

* bump minimum required cryptography version to 3.3

* drop support for Python 3.5

* use binary wheels for cryptography

* Revert "use binary wheels for cryptography"

This reverts commit 91a04c612ed1d0dd9fd541dfefe21cac7c25b1c1.

* docker ci: compile cryptography with rust

* check return code of SSL_CTX_set_alpn_protos, fix #992

* paint it black!

* fix line lengths as well :upside_down_face:

* 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>

* 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

* Drop CI for OpenSSL 1.0.2

* Delete code for coverage reasons

* Bump minimum cryptography version

* Context.set_verify: allow omission of callback

* squeeze to 80 chars

* make it clear that default callback is used