bpo-34523: Use _PyCoreConfig instead of globals

[vstinner]

Get configuratiom the core configuration of the interpreter, rather
than getting global configuration variables. For example, replace
Py_QuietFlag with core_config->quiet.

https://bugs.python.org/issue34523

[bmwiedemann]

Unfortunately, the description does not tell much about "Why this is good?" (edit: link went to different PR)
Does it help reduce unexpected influences from other places in code?

[vstinner]

Unfortunately, the description does not tell much about "Why this is good?" (edit: link went to different PR)
Does it help reduce unexpected influences from other places in code?

For the long term, I plan to deprecate global variables, especially global configuration variables like Py_QuietFlag. Moreover, I would like to support running two Python interpreters inside the same process but with two different configurations.

See https://www.python.org/dev/peps/pep-0554/ for more information about Eric Snow's project @ericsnowcurrently.