Python 3.6.x apparently does not properly implement PEP-0221, so `import
theano.tensor as tt` does not behave the same as `from theano.tensor as tt` and
causes the former to fail for circular imports.

This problem does not exist in Python >= 3.7.

The relevant Python issues are
- https://bugs.python.org/issue23203
- https://bugs.python.org/issue30024