Why there are errors of 'ERROR tests/distributions_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)

**Describe the bug**
When I try to run the unit tests, there many errors like:

ERROR tests/distributions_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)
ERROR tests/estimators_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)
ERROR tests/interp_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)
ERROR tests/light_curves_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)
ERROR tests/orbits/keplerian_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)
ERROR tests/orbits/simple_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)
ERROR tests/orbits/ttv_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)
ERROR tests/units_test.py - AttributeError: partially initialized module 'theano' has no attribute 'gof' (most likely due to a circular import)
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 9 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!**To

Reproduce**
This section should include a simple, standalone code snippet that demonstrates the issue.

**Expected behavior**
A clear and concise description of what you expected to happen.
How can I pass the tests?
**Your setup (please complete the following information):**
- Version of exoplanet: Successfully installed exoplanet-0.6.1.dev18+g6161d2b
- - Operating system: Mac
- Python version & installation method (pip, conda, etc.): python -m pip install -e .

**Additional context**
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