20210325

• nis extension is no longer built on Linux. This extension was adding a dependency on libnsl, which isn't part of the Linux Standard Base. Modern Linux distributions are dropping libnsl from the base install, leading to missing libraries at run-time. See discussion in #51 for more.
• macOS Python 3.8 is now built against the 10.15 SDK instead of 11.x.
• PYTHON.json version incremented to 7 to add Apple SDK metadata.
• musl libc upgraded to 1.2.2.
• setuptools upgraded to 54.1.1.
• LibreSSL upgraded to 3.2.5.
• OpenSSL upgraded to 1.1.1k.
• SQLite upgraded to 3.35.2.