chore: [Deprecation] Use Markup instead of HTMLString

[ghost]

Description

Bump up requests to 2.26.0
Move from HTMLString to Markup due deprecation warnings

ADDITIONAL INFORMATION

• Has associated issue:

• Unable to install dev/test dependencies as per CONTRIBUTING #1721
• DeprecationWarning: 'HTMLString' will be removed in WTForms 3.0 #1692

[codecov]

Codecov Report

Merging #1729 (8f7204c) into master (0747f7d) will increase coverage by 0.00%.
The diff coverage is 70.00%.

@@           Coverage Diff           @@
##           master    #1729   +/-   ##
=======================================
  Coverage   76.80%   76.80%           
=======================================
  Files          56       56           
  Lines        8113     8115    +2     
=======================================
+ Hits         6231     6233    +2     
  Misses       1882     1882           

Flag	Coverage Δ
python	76.80% <70.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
flask_appbuilder/fieldwidgets.py	63.15% <66.66%> (+0.32%)	⬆️
flask_appbuilder/upload.py	36.63% <75.00%> (+0.63%)	⬆️

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