Support latest drf@3.11.0 and django@3.0.0 from Betalos master

[uoxiu]

Improve changes from PR #274

[codecov]

Codecov Report

Merging #275 into master will increase coverage by 0.03%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
+ Coverage   89.28%   89.31%   +0.03%     
==========================================
  Files           8        8              
  Lines         933      936       +3     
==========================================
+ Hits          833      836       +3     
  Misses        100      100

Impacted Files	Coverage Δ
rest_framework_mongoengine/repr.py	89.88% <0%> (-0.12%)	⬇️
rest_framework_mongoengine/serializers.py	93.64% <100%> (-0.02%)	⬇️
rest_framework_mongoengine/fields.py	86.75% <100%> (ø)	⬆️
rest_framework_mongoengine/routers.py	100% <100%> (ø)	⬆️
rest_framework_mongoengine/validators.py	85.1% <94.11%> (+1.77%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 95c3fe8...95d93ab. Read the comment docs.