v0.16.3 (2021-09-21)

• clang/llvm: fix version detection (#19978)
• Fix use of quotes in Python build system (#22279)
• Cray: fix extracting paths from module files (#23472)
• Use AWS CloudFront for source mirror (#23978)
• Ensure all roots of an installed environment are marked explicit in db (#24277)
• Fix fetching for Python 3.8 and 3.9 (#24686)
• locks: only open lockfiles once instead of for every lock held (#24794)
• Remove the EOL centos:6 docker image