pyoxidizer: update README.md with several small fixes Currently, pyoxidizer.bzl does not mention the git commit that should be checked out, so these instructions are a bit difficult to follow right now (impossible, technically), so I removed the instruction to `git checkout <Git commit>` and the admonition to use a specific version of PyOxidizer. I don't even know if the project currently builds with the "0.7.0-pre" version that was previously recommended. As fallout from that change to not "pin" to a specific PyOxidizer, I had to update the Python version to use when running the tests. While here, I added a recommendation to use `--release`, as the primary reason for this project is performance, and it may have been leaving some on the table to not have that there. Differential Revision: https://phab.mercurial-scm.org/D11698