In #4220, I switched BIO_new to take a const pointer (true in 1.1.0, but
not 1.0.2) on grounds that:

    This also folds in the const bits from 1.1.0, on the assumption that,
    now that the function pointer check is gone, it will just cause cffi to
    generate more conservative pointer types that work for 1.0.2 as well.

But I got this backwards. If the bindings say BIO_METHOD*, cffi will
pass a BIO_METHOD* to BIO_new, which works in both OpenSSL versions. If
it says const BIO_METHOD*, cffi will pass const BIO_METHD* to BIO_new,
which does not work in 1.0.2.

(Although cryptography.io's build ignores all these warnings anyway, so
it's kind of moot.)

* libre 2.7.3 compatibility

* add a changelog

* actually build against 2.7.3

* Remove defines for openssl 1.1.0 pre

* Update bio.py

* Update dh.py

* Update dsa.py

* Update rsa.py

* Update x509_vfy.py

* Compress branches

* Remove unused BIO bindings.

This also folds in the const bits from 1.1.0, on the assumption that,
now that the function pointer check is gone, it will just cause cffi to
generate more conservative pointer types that work for 1.0.2 as well.

* Restore some functions used externally.

Datagram BIO_CTRL_* constants are intentionally omitted per discussion
on the PR.

* + more DTLS bindings

* + BIO_CTRL_DGRAM*

* + read ahead functions

* rm BIO_CTRL_DGRAM_SET_PEEK_MODE

* rm BIO_CTRL_DGRAM_SET_DONT_FRAG

* + link mtu conditional logic

* rm some BIO_CTRL_DGRAM* bindings

* No more FUNCS/MACROS distinction

* change the docs to not talk about MACROS since they're gone

* remove out of date comment

* Attempt to simplify the libressl checing

* SHENANGINS

* Attempted fix

* More simplification

refs #3248