disable foreign key checks when dropping tables on MySQL

[skarzi]

Closes #122

This is a quick solution for problems with MySQL and drop_models_tables. It's not an ideal solution, better one (but also with more time consuming and verbose implementation) is described in #122 (comment)

On branch test-mysql-drop-table-fix you can find this branch merged to branch from #129, where all tests are running against MySQL (started in a container), so we can see this solves #122 - https://github.com/wemake-services/django-test-migrations/actions/runs/389114796

[codecov]

Codecov Report

Merging #149 (8459985) into master (f6958d5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #149   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines          327       329    +2     
  Branches        33        34    +1     
=========================================
+ Hits           327       329    +2     

Impacted Files	Coverage Δ
django_test_migrations/sql.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 f6958d5...8459985. Read the comment docs.