Most of the bugbears involved `assert False` and bad function keyword defaults.
In tests, the former often signaled needed `pytest.raises` rewrites.

Closes #117.

This reverts commit 19a336f4.

* add codecov

* fix coverage report generation by pytest

Misc imports and tests refactoring

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

Indirect references to theano.tensor imports at module level were also converted
to direct references in quite a few cases.