bpo-40334: Allow to run make regen-pegen without distutils

[pablogsal]

https://bugs.python.org/issue40334

[pablogsal]

CC: @lysnikolaou

[pablogsal]

Sadly, we cannot support < Python3.8 because the walrus operator is used in the generated meta-parser :(

@serhiy-storchaka Would you like to skip the pegen regeneration if PYTHON_FOR_REGEN is not at least 3.8? Or maybe crash with a better error message?

[lysnikolaou]

LG.

[pablogsal]

There isn't much that we can do without a considerable redesign to allow running this with something lower than Python3.8 as the generated parser for Python heavily uses the := operator so I will land this so the rest of the reported problems are fixed and a clear error message is emitted in case you run with something lower than 3.8.