release 5.1.0

Features:

• issue #140: expose internal constants to allow changing default
  timeouts and polling intervals.
• issue #141: add PID of expired process to ProcessExpired
  exception. Credits: @rsiemens
• Add pool decorators parameter allowing to provide a PebblePool
  as executor for the decorated functions.

Fixes:

• issue #137: reset SIGTERM default handler to child processes.
• Include concurrent and asynchronous modules as top-level
  packages.

Improvements:

• Improve type hinting logic. Credits: @emiliadecaudin , @ccazabon
• Refactor internal logic: remove code duplication.