Ensure that API tracer returns the right value types

[Oberon00]

See https://github.com/open-telemetry/opentelemetry-python/pull/141/files#r325107543.

Also check mypy configuration to see why this is missed.

[mauriciovasquezbernal]

I found out that mypy doesn't comply about a missing return statement when the function is empty: http://mypy-lang.blogspot.com/2016/11/mypy-046-released.html, not sure if there is a way to enable that, probably not.

[c24t]

We need unit tests that have well-defined -- and different! -- behavior in the case that only the API is loaded and in the case that the SDK is loaded. This could look like the example app.

See also #166. If the API ever returns instances of "interface" classes we are guaranteed to crash. One argument for using ABCs.

[codeboten]

There's an open issue in mypy to handle empty functions with None returns python/mypy#2350