Show deprecation warnings

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• viewcode_import and sphinx.application.CONFIG_FILENAME has been marked as deprecated. But no warnings are emitted for them.

[codecov]

Codecov Report

Merging #6224 into 2.0.1 will decrease coverage by <.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##            2.0.1    #6224      +/-   ##
==========================================
- Coverage   83.55%   83.54%   -0.01%     
==========================================
  Files         272      272              
  Lines       39349    39354       +5     
  Branches     5867     5868       +1     
==========================================
+ Hits        32878    32879       +1     
- Misses       5124     5126       +2     
- Partials     1347     1349       +2

Impacted Files	Coverage Δ
sphinx/application.py	77.35% <100%> (+0.06%)	⬆️
sphinx/ext/viewcode.py	80.76% <100%> (+0.12%)	⬆️
tests/test_ext_inheritance_diagram.py	100% <100%> (ø)	⬆️
sphinx/util/osutil.py	70.74% <20%> (-1.48%)	⬇️
sphinx/testing/fixtures.py	85.32% <0%> (-0.92%)	⬇️

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