when imported, instead doing it at build time.  This makes importing
  sysconfig faster and reduces Python startup time by 20%.

they work properly on systems configured with large pipe buffers.

cStringIO

Structured Exception Handling on windows.

  using the same sys.flags as the current process.
Backport from default branch.

- Issue #17477: Update the bsddb module to pybsddb 5.3.0, supporting
  db-5.x, and dropping support for db-4.1 and db-4.2.

  db-5.x, and dropping support for db-4.1 and db-4.2.

when it needs support from timemodule (which is a .so on linux):
link in timemodule.c for the required functions.