Mypy type checking

[JohnGiorgi]

Overview

This PR sets up static type checking with MyPy as part of the CI/CD.

We still have 13 errors to solve:

geneeval/data.py:27: error: Incompatible return type for "__new__" (returns "PreprocessedData", but must return a subtype of "DatasetReader")
geneeval/classifiers/auto_classifier.py:9: error: Incompatible return type for "__new__" (returns "SupervisedClassifier", but must return a subtype of "AutoClassifier")
geneeval/engine.py:28: error: Incompatible types in assignment (expression has type "DatasetReader", variable has type "PreprocessedData")
geneeval/engine.py:30: error: Incompatible types in assignment (expression has type "AutoClassifier", variable has type "SupervisedClassifier")
tests/test_data.py:25: error: "DatasetReader" has no attribute "X_train"
tests/test_data.py:29: error: "DatasetReader" has no attribute "lb"
tests/test_data.py:29: error: "DatasetReader" has no attribute "y_train"
tests/test_data.py:36: error: "DatasetReader" has no attribute "X_test"
tests/test_data.py:37: error: "DatasetReader" has no attribute "lb"
tests/test_data.py:37: error: "DatasetReader" has no attribute "y_test"
tests/test_data.py:38: error: "DatasetReader" has no attribute "splits"
geneeval/fetcher/auto_fetcher.py:17: error: Incompatible return type for "__new__" (returns "Fetcher", but must return a subtype of "AutoFetcher")
geneeval/main.py:29: error: "AutoFetcher" has no attribute "fetch"
Found 13 errors in 6 files (checked 25 source files)

Most of these stem from our use of __new__, which appears to confuse mypy (see python/mypy#1021, python/mypy#794). I think it is because __new__ is supposed to return an instance of the class it is called from (or a subclass).