Remove support for buildPython versions 3.5 and 3.6

[mhsmith]

See product/gradle-plugin/README.md for checklist.

3.5 was desupported by:

• CPython in 2020-09
• pip in 2021-01
• Debian in 2022-06

3.6 was desupported by:

• CPython in 2021-12
• pip in 2022-01
• It was never included in a stable version of Debian.

[mhsmith]

Once we no longer support 3.5, the special case for execResult.exitValue == 103 can be removed from PythonPlugin.execBuildPython, and test_py_not_found can changed to expect the message printed by the launcher itself, or perhaps simply removed.

[mhsmith]

Enabling PEP 517 (#715) required us to backport pip's vendored copy of tomli in order to support TOML 1.0 syntax. But this version of tomli contains type annotations on local variables, which were added in Python 3.6. So we need to desupport 3.5, and we might as well remove 3.6 at the same time.