2.1.0rc0

Python 3.9 (#1240)

* Python 3.9

* pip-compile --output-file=test_requirements.txt test_requirements.in

* Fix test_into_definition

* Fix error in test_serializer_into_definition.py

* types-setuptools

* types-requests

* types-pytz

* types-mock

* Fix examples/Pipelines-with-Gordo.ipynb

* import_locate refactoring

* import_locate refactoring

* sklearn.linear_model.LinearRegression

* Black reformating