centralize specification of python versions

Since we allow users to provide their own `environment.yml` (#31), they can also specify a python version and we [require that version to be aligned](https://github.com/brain-score/core/blob/65a7d174a022307b83aea66d68bb26c022ef751d/brainscore_core/plugin_management/test_plugins.py#L63) with the library's supported versions.

We also specify the python version in the [pyproject.toml](https://github.com/brain-score/core/blob/16ffe02bc250c26ee60bde48794a1c21c10ab9eb/pyproject.toml#L12), [.readthedocs.yml](https://github.com/brain-score/core/blob/16ffe02bc250c26ee60bde48794a1c21c10ab9eb/.readthedocs.yml#L2), and the [.travis.yml](https://github.com/brain-score/core/blob/16ffe02bc250c26ee60bde48794a1c21c10ab9eb/.travis.yml#L2).
(Also [some](https://github.com/brain-score/core/blob/16ffe02bc250c26ee60bde48794a1c21c10ab9eb/tests/test_plugin_management/test_conda_score__brainscore_dummy/pyproject.toml#L6) [tests](https://github.com/brain-score/core/blob/16ffe02bc250c26ee60bde48794a1c21c10ab9eb/tests/test_plugin_management/test_test_plugins__brainscore_dummy/pyproject.toml#L6) but I think those are fine.)

Does it make sense to unify the python version specification? Or is this acceptable?