Fix broken tests

[s-weigand]

The tests are broken for two reasons:

• pytest==6.1.0 is incompatible with pytest-rerunfailures<=9.1
• djangocms-text-ckeditor>=4.0.0 is incompatible with django-cms<3.8

[codecov]

Codecov Report

Merging #109 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #109   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           70        71    +1     
  Branches         6         6           
=========================================
+ Hits            70        71    +1     

Impacted Files	Coverage Δ
...s_equation/templatetags/djangocms_equation_tags.py	100.00% <0.00%> (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 5b735c2...ffb4664. Read the comment docs.