python: Support future python version up to 3.9
This change fixes automake bug#28160.
Since AM_PYTHON_PATH macro takes no maximum version argument, there is
no need to generate _AM_PYTHON_INTERPRETER_LIST dynamically, like what
was previously done by the reverted commit
1d60fb72168e62d33fe433380af621de64e22f23. We could rely on M4 to
generate this list statically however this is likely to be a complex
solution that would not improve maintainability.
* m4/python.m4 (_AM_PYTHON_INTERPRETER_LIST): Add 'python3.7',
'python3.8', and 'python3.9'.
* NEWS: Update.