Add typehints to the Python speechd API

**Is your feature request related to a problem? Please describe.**

Currently, the speech dispatcher Python client does not include typehints. This prevents static analysis tools such as mypy from verifying correct usage of the library from outside, as well as the internal integrity.

**Describe the solution you'd like**

Add typehints to all external/internal methods and structures.

**Describe alternatives you've considered**

The module can be excluded from statical analysis by the developer, however adding the types is the preferred solution.

**Additional context**

I can take this on, since it relates to a project I'm currently working on. Just wanted to check with the project whether we're of the same mind, before starting the actual work. :)