Drop support for Python <= 3.7

[viniciuschiele]

• The current version will require Flask >= 2.2
• Most recent versions of Flask only support Python >= 3.8
• Drop Python 2 and Python <= 3.7 to make the code more maintainable

[codecov]

Codecov Report

Patch coverage: 95.00% and project coverage change: +0.17 🎉

Comparison is base (b34362d) 91.64% compared to head (3c0484f) 91.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   91.64%   91.82%   +0.17%     
==========================================
  Files           6        6              
  Lines         431      428       -3     
==========================================
- Hits          395      393       -2     
+ Misses         36       35       -1     

Impacted Files	Coverage Δ
flask_apscheduler/utils.py	91.11% <87.17%> (+0.78%)	⬆️
flask_apscheduler/api.py	83.05% <100.00%> (ø)
flask_apscheduler/auth.py	87.50% <100.00%> (ø)
flask_apscheduler/json.py	92.85% <100.00%> (ø)
flask_apscheduler/scheduler.py	99.38% <100.00%> (ø)

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