Closes #2155: Support code directive

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• refs: Use reStructuredText's code directive #2155
• It would be better to integrate code directive with code-block directive. But we need to have deeper discussion about it. So this only resolve that code directive has been not working in Sphinx. Let's discuss about integration in another topic.
• Note: sourcecode directive still has been incompatible with docutils'. In Sphinx, it is an alias of code-block directive (since its birth, f2ff041). On the other hand, in docutils, it is an alias of code directive.

[tk0miya]

I did not update our document. I suspect two "code block" directives will confuse users. This change is only needed for compatibility with standard reStructuredText, I believe.

[codecov]

Codecov Report

Merging #6132 into 2.0 will increase coverage by 0.01%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##              2.0    #6132      +/-   ##
==========================================
+ Coverage   82.71%   82.72%   +0.01%     
==========================================
  Files         272      274       +2     
  Lines       39214    39269      +55     
  Branches     5856     5861       +5     
==========================================
+ Hits        32434    32487      +53     
- Misses       5447     5448       +1     
- Partials     1333     1334       +1

Impacted Files	Coverage Δ
tests/test_directive_other.py	100% <100%> (ø)	⬆️
sphinx/testing/restructuredtext.py	100% <100%> (ø)
tests/test_directive_patch.py	100% <100%> (ø)
sphinx/directives/patches.py	94.82% <100%> (+1.21%)	⬆️
sphinx/directives/__init__.py	89.18% <80%> (-1.2%)	⬇️

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[tk0miya]

I'm merging this now. Please let me know if you have comment.