Improve docs for Python 3.7+

[pzrq]

This PR modifies the README.rst and tutorials to:

• make Python 3.7 support more explicit
• drop an old reference to "$(which python3)" noting also that the supported Python versions are already present earlier in the README.rst file's prerequisites section (as an aside which usage could be improved with command -v)
• document the caveat that test.pystone was removed from the standard library in 3.7
• add the suggestion to check out pyperf and pyperformance as an aside to tutorial 1

PR Checklist:

• [x] All new features have been tested (N/A)
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct