Link deprecation docs pytest.raises 'message' warning

[nicoddemus]

As commented in #3974 (comment)

[ndevenish]

Looks good - my one (minor) concern was linking to /latest/ - but it looks like you've accounted for that and old deprecations are kept listed on that page even after removal.

although it does look as though the anchor links change (everything before "Removed Features" has a deprecations.html#named-title-anchor whereas after has #id<num>), this is probably just a manual page-generation issue? And anyway, getting onto the right page is probably good enough.

From unfortunate experience, I know that some people remain stuck to old versions for quite some time, especially if v5 is dropping python 2.7.

[nicoddemus]

my one (minor) concern was linking to /latest/

Great catch. I've changed the link to point to the 4.6-maintenance release docs instead. 👍

That link should remain valid and unchanged for a long time.

[codecov]

Codecov Report

Merging #5421 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5421   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files         114      114           
  Lines       25502    25502           
  Branches     2479     2479           
=======================================
  Hits        24479    24479           
  Misses        717      717           
  Partials      306      306

Impacted Files	Coverage Δ
src/_pytest/deprecated.py	100% <ø> (ø)	⬆️

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[asottile]