Refactor modules

[mdumandag]

There were lots of tiny modules, which is not idiomatic Python. I have replaced most of those, and simplified the module structure of the client a lot. Most of these are simple moves without a code change.

Apart from that, I have applied isort and black to whole code base. I plan to add them to CI in the short term as well, in another PR.

Below, I list other small changes I did as part of this PR:

• Moved types that are mentioned in the public APIs to typing.py
• Renamed UpstashException to UpstashError, as the errors that originate from the Exception class are prefixed with Error throughout the standard library and ecosystem.
• Converted the type of the rest_encoding parameter from boolean | Literal["base64"] to None | Literal["base64"]. I believe it conveys the message better.
• Removed the mix-in class FormattedResponse altogether. It was not providing any benefit other than providing the dict of formatters. I converted it into a simple dictionary.
• Converted the rest_retry_interval parameter into a float from int. Time related parameters are generally given as floats that represent seconds. It also makes more sense, as you now able to provide more granular times.
• Removed CommandsProtocol altogether. That abstraction was not used at all.
• Removed some commented out codes. I don't like the idea of shipping dead code, we can implement them properly later.