py-django-extensions: updated to 2.2.6

2.2.6
Changes:
- Improvement: travis, update pypy and pypy3 versions
- Improvement: shell_plus, ability to print location/traceback besides sql
- Improvement: runserver_plus, ability to print location/traceback besides sql
- Improvement: UniqueFieldMixin, Support Django 2.2 UniqueConstraint.condition
- Improvement: DEFAULT_MYSQL_ENGINES, add mysql.connector.django
- Improvement: shell_plus, allow setting SHELL_PLUS="notebook"
- Improvement: shell_plus, add -c/--command to shell_plus mirroring django's shell command
- Fix: shell_plus, fix postgresql debug wrapper on django 3.0 or higher
- Fix: runserver_plus, fix postgresql debug wrapper on django 3.0 or higher

(adam)