fix format of nested dataclasses

[aliereno]

Fixes #98

[codecov]

Codecov Report

Merging #99 (da5eccb) into master (1e3e942) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #99   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          517       516    -1     
  Branches        86        86           
=========================================
- Hits           517       516    -1     

Impacted Files	Coverage Δ
devtools/prettier.py	100.00% <100.00%> (ø)

---

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 1e3e942...da5eccb. Read the comment docs.

[samuelcolvin]

Thanks so much for this, so sorry for the delay in getting around to it.