https://github.com/django-dbbackup/django-dbbackup/issues/314

This reverts merge request !61

 * Update django-two-factor-auth to current release
 * Allow phunnumber-field 3.x or 4.x while django-two-factor-auth
   still crunch https://github.com/Bouke/django-two-factor-auth/pull/326
 * Run poetry update
   * Update django-image-cropping and easy_thumbnails for DJango 3.0 support
     (cf. !61 and #75)

This is due to a version with the fixed dependency not being released yet.

Revert this commit once something newer than 1.9.1 gets released to PyPI.

This reverts commit 757abb1d.

Rationale: Adding logic to decide on the login view to use, and
reliably disable the default login view if 2fa is enabled, adds
unnecessary complexity, compared to django-two-factor-auth being
a quite lightweight dependency.