Bugfixes
* SUSHI: Deal with missing institutional identifier for customer. [Geoffrey Spear]
(see Issue 72. This should catch the exception involved in the
issue, but there may be a wider issue with the Gale report since
the "customer" object involved should not be None if there's
an actual report to read, since the report itself is in the
Customer XML element.)
Code quality/CI
* Add pre-commit black hook, format all code with black. [Geoffrey Spear]
* Python 3.7 support. [Geoffrey Spear]
* Replace arrow library with pendulum. [Geoffrey Spear]
* Pyup: try updating all deps. [Geoffrey Spear]