Enable mypy type checking

[delucchi-cmu]

While updating copier, I tried to enable type checking, but ran into some issues. I've disabled it for now, but the issues should be addressed so we can have type-checking going forward.

1 - Multiple conftest files

Initial module discovery is unhappy that we have multiple files named conftest.py

$ mypy ./src ./tests --ignore-missing-imports
tests/hipscat/io/conftest.py: error: Duplicate module named "conftest" (also at "tests/hipscat/catalog/conftest.py")
tests/hipscat/io/conftest.py: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#mapping-file-paths-to-modules for more info
tests/hipscat/io/conftest.py: note: Common resolutions include: a) using `--exclude` to avoid checking one of them, b) adding `__init__.py` somewhere, c) using `--explicit-package-bases` or adjusting MYPYPATH
Found 1 error in 1 file (errors prevented further checking)

Can address by

• write fixtures in separate files for code organization, and import into a single conftest.py
• combining all fixtures into single conftest.py
• only checking the ./src directory

2 - Warnings:

$ mypy ./src --ignore-missing-imports
src/hipscat/io/paths.py:50: error: Argument 1 to "int" has incompatible type "Optional[int]"; expected "Union[str, bytes, bytearray, memoryview, array[Any], mmap, _CData, PickleBuffer, SupportsInt, SupportsIndex, SupportsTrunc]"  [arg-type]
src/hipscat/io/file_io/file_io.py:93: error: Incompatible default for argument "metadata_collector" (default has type "None", argument has type "List[Any]")  [assignment]
src/hipscat/io/file_io/file_io.py:93: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
src/hipscat/io/file_io/file_io.py:93: note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
src/hipscat/catalog/pixel_node.py:55: error: Cannot determine type of "hp_order"  [has-type]
src/hipscat/catalog/pixel_node.py:62: error: Need type annotation for "children" (hint: "children: List[<type>] = ...")  [var-annotated]
src/hipscat/catalog/pixel_node.py:108: error: Need type annotation for "leaf_descendants" (hint: "leaf_descendants: List[<type>] = ...")  [var-annotated]
src/hipscat/pixel_math/pixel_margins.py:21: error: Need type annotation for "_suffixes" (hint: "_suffixes: Dict[<type>, <type>] = ...")  [var-annotated]
src/hipscat/io/write_metadata.py:136: error: Need type annotation for "hips_structure" (hint: "hips_structure: Dict[<type>, <type>] = ...")  [var-annotated]
src/hipscat/catalog/catalog_parameters.py:14: error: Incompatible default for argument "input_paths" (default has type "None", argument has type "List[str]")  [assignment]
src/hipscat/catalog/catalog_parameters.py:14: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
src/hipscat/catalog/catalog_parameters.py:14: note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
src/hipscat/catalog/catalog.py:11: error: Incompatible default for argument "catalog_path" (default has type "None", argument has type "str")  [assignment]
src/hipscat/catalog/catalog.py:11: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
src/hipscat/catalog/catalog.py:11: note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
Found 9 errors in 7 files (checked 19 source files)