Pin sphinx to version 1.8.5

[santisoler]

New versions of Sphinx (2.0.0, 2.0.1) create unexpected wrong format on docstrings.
A momentarily solution is to pin Sphinx to the last version before 2.0.0: 1.8.5.
We should go back to the latest Sphinx release when this bug is fixed on their
repository.

This issue was raised on fatiando/harmonica#58.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.