ref: use absolute imports for BaseManager

[asottile-sentry]

fixing an __init__.py cycle one step at a time

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.98%. Comparing base (5d711ca) to head (092daf4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #72965   +/-   ##
=======================================
  Coverage   77.97%   77.98%           
=======================================
  Files        6616     6615    -1     
  Lines      295310   295343   +33     
  Branches    50887    50887           
=======================================
+ Hits       230280   230310   +30     
- Misses      58739    58740    +1     
- Partials     6291     6293    +2     

Files	Coverage Δ
src/sentry/db/models/__init__.py	100.00% <ø> (ø)
src/sentry/db/models/base.py	91.32% <100.00%> (-0.04%)	⬇️
src/sentry/db/models/outboxes.py	81.10% <100.00%> (+0.08%)	⬆️
src/sentry/db/models/paranoia.py	100.00% <100.00%> (ø)
src/sentry/discover/models.py	94.31% <100.00%> (+0.06%)	⬆️
src/sentry/incidents/models/alert_rule.py	95.38% <100.00%> (ø)
.../sentry/incidents/models/alert_rule_activations.py	93.47% <100.00%> (ø)
src/sentry/incidents/models/incident.py	98.00% <100.00%> (ø)
src/sentry/models/activity.py	90.74% <100.00%> (+0.08%)	⬆️
src/sentry/models/apiapplication.py	95.18% <100.00%> (+0.05%)	⬆️
... and 43 more

... and 6 files with indirect coverage changes