Integration with aiopg 1.3.0

Is your feature request related to a problem?
Aiopg 1.3.0 has a breaking change on its internal Context Manager class.
The multiple classes were centralized into a single class called _ContextManager.
The constructor for _ContextManager was also changed accordingly.
Therefore, in order to integrate with aiopg 1.3.0, a further work is required.
Currently, PR #560 has forced the instrumentor not to instrument aiopg 1.3.0.
Before fixing the integration, the restriction will not be released.

Describe the solution you'd like
The best solution here will be a conditional import to support both aiopg 1.3.0 and aiopg < 1.3.0.
The conditional import can address the difference between 1.3.0 and < 1.3.0 in Context Manager and reconstruct the same property object.
The main logic then will share that object to run the code.
A set of new test cases under aiopg instrumentors should also be created to test against 1.3.0 and < 1.3.0.

Start in the aiopg instrumentors and inspect the existing Context Manager integration, then review PR #560 and the aiopg version handling. Add coverage for both aiopg 1.3.0 and earlier versions, and confirm the instrumentor works without the current version restriction.