Support the PYTHONVERBOSE environment variable

For some performance-relevant work I need to do, it'd make things more doable to have PyOxidizer-built binaries respect the PYTHONVERBOSE variable (which is equivalent to `python -v` if `PYTHONVERBOSE=1`, `python -v -v` for `PYTHONVERBOSE=2`, etc). This is a feature request so I don't lose track of wanting this.

The issue names no files or tests. Start by tracing how PyOxidizer-built binaries handle environment variables, then compare PYTHONVERBOSE=1 and =2 with Python's -v and -v -v behavior. Done means the binaries respect those variable values consistently.

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