Refactor library.py to use NamedTuple, fix mypy errors

[tianyizheng02]

Contributes to #45 by fixing existing type hint errors prior to introducing mypy to the repo.

Refactor library.py to store API output using NamedTuples rather than plain Python dicts. This makes the output more user-friendly, less error-prone, and easier to type-hint.

Before:

> mypy --strict pittapi/library.py
pittapi/__init__.py:23: error: Skipping analyzing "gevent": module is installed, but missing library stubs or py.typed marker  [import-untyped]
pittapi/__init__.py:23: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
pittapi/library.py:48: error: Function is missing a return type annotation  [no-untyped-def]
pittapi/library.py:48: note: Use "-> None" if function does not return a value
pittapi/library.py:86: error: Call to untyped function "HTMLStrip" in typed context  [no-untyped-call]
pittapi/library.py:145: error: Value of type "Response" is not indexable  [index]
pittapi/library.py:156: error: Value of type "Response" is not indexable  [index]
Found 5 errors in 2 files (checked 1 source file)

After:

> mypy --strict pittapi/library.py       
pittapi/__init__.py:23: error: Skipping analyzing "gevent": module is installed, but missing library stubs or py.typed marker  [import-untyped]
pittapi/__init__.py:23: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

Like in #193, the remaining missing-imports error will need to be ignored with --ignore-missing-imports because gevent simply doesn't provide any type hints for mypy.