stake-pool-py: Clean up dependencies

[joncinque]

Problem

We want to separate the dependencies to avoid bundling too much in the stake pool python package.

Solution

Split up the deps between requirements.txt and optional-requirements.txt. @c3pko You can use everything in requirements.txt as the dependencies, and everything in optional-requirements.txt in `optional-dependencies. After this is merged, you can rebase your PR, and remove both of these requirements files.

[joncinque]

The stake pool python tests passed, and that's what matters most -- merging on red!