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
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