Remove factory-boy version constraint.

[mblayman]

PR #407 is failing CI currently because of a package version issue and the PR is only changing a typo in the usage file. Some dependency requirement must have changed recently.

I'm trying this PR to see if it's now possible to have factory-boy working on a newer version.

[codecov-io]

Codecov Report

Merging #408 into master will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #408      +/-   ##
==========================================
- Coverage   91.93%   91.79%   -0.14%     
==========================================
  Files          55       55              
  Lines        2925     2925              
==========================================
- Hits         2689     2685       -4     
- Misses        236      240       +4

Impacted Files	Coverage Δ
setup.py	80% <ø> (ø)	⬆️
example/models.py	91.2% <0%> (-4.4%)	⬇️

---

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 2ef94c7...137e0c8. Read the comment docs.

[mblayman]

This makes the test suite green again so I'm going to go ahead and merge it.