Fix #6245: circular import error on importing SerializingHTMLBuilder

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• refs: import sphinxcontrib.serializinghtml fails with circular import woes #6245

[codecov]

Codecov Report

Merging #6249 into 2.0.1 will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            2.0.1    #6249      +/-   ##
==========================================
- Coverage   83.54%   83.51%   -0.03%     
==========================================
  Files         272      274       +2     
  Lines       39354    39391      +37     
  Branches     5868     5871       +3     
==========================================
+ Hits        32879    32899      +20     
- Misses       5126     5140      +14     
- Partials     1349     1352       +3

Impacted Files	Coverage Δ
sphinx/builders/html.py	81.78% <100%> (-0.03%)	⬇️
sphinx/builders/dirhtml.py	92.59% <100%> (+0.59%)	⬆️
sphinx/builders/singlehtml.py	93.51% <100%> (+0.12%)	⬆️
sphinx/__init__.py	73.91% <0%> (ø)
sphinx/make_mode.py	0% <0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d34302a...9dfbfbf. Read the comment docs.