Fix mypy errors and rename NamedTuple fields in gym.py

[tianyizheng02]

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

Run mypy with --strict flag and fix resulting mypy errors in gym.py.

Rename some fields in the Gym NamedTuple class in order to clarify their meanings and to fix a mypy error. The count field specifically causes a mypy error because mypy interprets the field's name as the count method that NamedTuple inherits from the base tuple class: According to the mypy devs, this is expected behavior and is not a bug:

• Better error message for incompatible assignment to method python/mypy#4507
• mypy sometimes confuses nested classes-in-functions which share the same name python/mypy#17047

Before:

> mypy --strict pittapi/gym.py        
pittapi/__init__.py:23: error: Skipping analyzing "gevent": module is installed, but missing library stubs or py.typed marker  [import-untyped]
pittapi/gym.py:22: error: Library stubs not installed for "bs4"  [import-untyped]
pittapi/gym.py:22: note: Hint: "python3 -m pip install types-beautifulsoup4"
pittapi/gym.py:22: note: (or run "mypy --install-types" to install all missing stub packages)
pittapi/gym.py:22: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
pittapi/gym.py:43: error: Incompatible types in assignment (expression has type "int", base class "tuple" defined the type as "Callable[[tuple[str | int, ...], Any], int]")  [assignment]
pittapi/gym.py:51: error: Argument "date" to "Gym" has incompatible type "None"; expected "str"  [arg-type]
pittapi/gym.py:51: error: Argument "count" to "Gym" has incompatible type "None"; expected "int"  [arg-type]
pittapi/gym.py:51: error: Argument "percentage" to "Gym" has incompatible type "None"; expected "int"  [arg-type]
Found 6 errors in 2 files (checked 1 source file)

After:

> mypy --strict pittapi/gym.py       
pittapi/__init__.py:23: error: Skipping analyzing "gevent": module is installed, but missing library stubs or py.typed marker  [import-untyped]
pittapi/gym.py:22: error: Library stubs not installed for "bs4"  [import-untyped]
pittapi/gym.py:22: note: Hint: "python3 -m pip install types-beautifulsoup4"
pittapi/gym.py:22: note: (or run "mypy --install-types" to install all missing stub packages)
pittapi/gym.py:22: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 2 errors in 2 files (checked 1 source file)

Like the remaining mypy errors from #193, we'll need to ignore these missing-imports errors with --ignore-missing-imports.