Add Django 4.0 support

[bence-the-great]

• force_text was removed in favor of force_str
• ugettext_lazy was removed in favor of gettext_lazy
• added Python 3.9 and 3.10 to the test matrix, because they're supported by Django 4.0 (while excluding Django versions, which do not support 3.9 and 3.10)

[codecov-commenter]

Codecov Report

Merging #387 (1b4456b) into master (c0612d6) will decrease coverage by 9.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
- Coverage   67.67%   58.43%   -9.25%     
==========================================
  Files          12       12              
  Lines         758      883     +125     
==========================================
+ Hits          513      516       +3     
- Misses        245      367     +122     

Impacted Files	Coverage Δ
django_elasticsearch_dsl/fields.py	55.50% <100.00%> (-5.96%)	⬇️
django_elasticsearch_dsl/documents.py	55.08% <0.00%> (-21.58%)	⬇️
django_elasticsearch_dsl/registries.py	57.79% <0.00%> (-16.38%)	⬇️
django_elasticsearch_dsl/__init__.py	90.90% <0.00%> (-9.10%)	⬇️
django_elasticsearch_dsl/test/testcases.py	96.29% <0.00%> (-3.71%)	⬇️
django_elasticsearch_dsl/apps.py	85.71% <0.00%> (-3.18%)	⬇️
...sticsearch_dsl/management/commands/search_index.py	39.03% <0.00%> (-3.16%)	⬇️
django_elasticsearch_dsl/signals.py	86.11% <0.00%> (-2.47%)	⬇️

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[safwanrahman]

Looks good to me. Thanks @bence92 🚀

[sondrelg]

Do you intend on releasing a patch version with these changes soon @safwanrahman? I see it was merged a few days ago 🙂