Use a context manager for Lock

We can save many lines, and so reducing the code complexity and readability, by implementing a context manager for the Lock base class.

The current code:

self._lock.acquire()
try:
    # ...
finally:
    self._lock.release()

Will be usable this way:

with self._lock:
    # ...

Pros:

• resources are always closed and garbaged;
• readability;
• less code;
• more pythonic;
• possibility to use the context manager or not;
• implementation time is quite small and not complex.

Cons:

• None.