* crypto._PassphraseHelper: pass non-callable passphrase using callback
Fixes #945

Before this commit, we would pass a bytes passphrase as a null terminated string.
This causes issue when a randomly generated key's first byte is null because
OpenSSL rightly determines the key length is 0.
This commit modifies the passphrase helper to pass the passphrase via the
 callback

* Update changelog to document bug fix

The additional certificates provided in the new `chain` parameter will be
untrusted but may be used to build the chain.

This makes it easier to validate a certificate against a store which
contains only root ca certificates, and the intermediates come from e.g.
the same untrusted source as the certificate to be verified.

Co-authored-by: Sandor Oroszi <sandor.oroszi@balabit.com>

Add X509Store.load_locations() to set a CA bundle file and/or an OpenSSL-
style hashed CA/CRL lookup directory, similar to the already existing
SSL.Context.load_verify_locations().

Co-authored-by: Sandor Oroszi <sandor.oroszi@balabit.com>

* Allow accessing a connection's verfied certificate chain

Add X509StoreContext.get_verified_chain using X509_STORE_CTX_get1_chain.
Add Connection.get_verified_chain using SSL_get0_verified_chain if
available (ie OpenSSL 1.1+) and X509StoreContext.get_verified_chain
otherwise.
Fixes #740.

* TLSv1_METHOD -> SSLv23_METHOD

* Use X509_up_ref instead of X509_dup

* Add _openssl_assert where appropriate

* SSL_get_peer_cert_chain should not be null

* Reformat with black

* Fix <OpenSSL.crypto.X509 object at 0x7fdbb59e8050> != <OpenSSL.crypto.X509 object at 0x7fdbb59daad0>

* Add Changelog entry

* Remove _add_chain

* simplify

* generate new certs and keys with 3072-bit RSA

* black

* add a test to avoid losing coverage

* make our CI less frustrating

* sigh, even less sensitive

* can we stop doing this on macos now?

From RFC 5280, section 4.1.2.9:

   [Extensions] MUST only appear if the version is 3 (Section 4.1.2.1).
   If present, this field is a SEQUENCE of one or more certificate
   extensions.  The format and content of certificate extensions in the
   Internet PKI are defined in Section 4.2.

X509 objects default to v1, so the test certs need a set_version(2) call. (Note
v3 is encoded as 2.)

* fix PKey.check for some broken keys

RSA_check_key is documented to return 1 for valid keys.
It (currently) returns 0 or -1 for invalid ones.
The previous code accepted invalid keys if RSA_check_key returns -1!

* add test