Fix mypy errors in dining.py

[tianyizheng02]

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

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

Before:

> mypy --strict pittapi/dining.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/dining.py:118: error: Function is missing a return type annotation  [no-untyped-def]
Found 2 errors in 2 files (checked 1 source file)

After:

mypy --strict pittapi/dining.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.