Build Tor without warnings or test failures with OpenSSL 3.0.0
According to the OpenSSL release strategy [release-strat] they're planning to release openssl 3.0.0 in early Q4 of this year.
Currently, many of the APIs that Tor uses are deprecated in OpenSSL 3.0.0-alpha [openssl-3]. It's still possible to build Tor with it, but you get a lot of deprecated-item warnings. We should fix those warnings before OpenSSL 3 is released.
Further, if we build without fatal warnings, there are some test failures. We should see if they are tor bugs or new openssl bugs, and fix them in the first case or report them in the second.
I don't think we necessarily need to backport this: OpenSSL 1.1 will be supported until 2023-09-11 [release-strat], whereas support for 0.3.5 is scheduled to end on 2020-02-02.