Make sure extra module imports are optional

[gbottari]

Prerequisites

• Can you reproduce the problem?
• Are you running the latest version?
• Did you check the documentation?
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Description

All extra modules in pyproject.toml (section [tool.poetry.extras]) must be truly optional. For instance, PollyTTS requires boto3. If a user doesn't have boto3 installed, they should be able to use other modules (e.g. MicrosoftTTS) without needing to install boto3.

The project already works with this in mind. However, there are no tests for this yet. Maybe we can use importlib to remove an engine dependency and check if importing the TTS engine (without instantiating it) works.

[willwade]

I have not touched tests (tsk tsk!) in my PR #25 - apologies. But I'll try too.. and try and remeber this

[willwade]

willwade@bfcaf29