Thread-safe atomic counters

Feature or enhancement

Python doesn't currently have a thread-safe atomic counter. CPython currently internally uses a few different patterns to implement thread-safe counters. For example, some test code uses appends to a shared list because the implementation of list.append() is atomic.

We should provide a counter that is explicitly thread-safe so that people don't have to rely on other idioms. The minimum initial API should provide:

• atomic increment
• get value (i.e., inspect)

The threading module may be a good place for it.

cc @rhettinger

Comienza revisando el módulo threading y los patrones existentes de contadores seguros para hilos descritos en el issue. Define y prueba la API mínima para el incremento atómico y la inspección del valor, incluido el comportamiento bajo acceso concurrente.