Prepare for 21.3 release

[ahopkins]

Waiting on #2042, #2053, #2055.

Includes changelog, some updates to the RTD templates, and bump version.

[codecov]

Codecov Report

Merging #2057 (91eb440) into master (6763e2b) will not change coverage.
The diff coverage is 100.000%.

@@            Coverage Diff            @@
##            master     #2057   +/-   ##
=========================================
  Coverage   92.184%   92.184%           
=========================================
  Files           38        38           
  Lines         3480      3480           
  Branches       582       582           
=========================================
  Hits          3208      3208           
  Misses         184       184           
  Partials        88        88           

Impacted Files	Coverage Δ
sanic/__version__.py	100.000% <100.000%> (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 6763e2b...91eb440. Read the comment docs.

[harshanarayana]

#2053, #2055 would be added to this list + signals when you merge them right ?

[ahopkins]

Yup.

[ashleysommer]

Late notice, but can we include #2059 too?

[ashleysommer]

Still needs #2042, #2053, #2055 descriptions added to the list

[ahopkins]

Still needs #2042, #2053, #2055 descriptions added to the list

Yup, that commit is coming soon.