Deprecate raises(..., 'code(as_a_string)') / warns(..., 'code(as_a_string)').

[asottile]

Resolves #4435

TODONE:

• resolve the CI failures (surely there's some!)
• write actual tests for the warning
• regendocs
• deprecation docs (by @nicoddemus 😁)

[asottile]

urgh and there's a circular import. great.

[RonnyPfannschmidt]

to highlight the importance of the call visually i'd propose a with statement in that case

[asottile]

if it's ok, I'm going to leave these as is so it's a less noisy diff -- we can clean these up in a followup

[codecov]

Codecov Report

Merging #4443 into features will decrease coverage by 53.44%.
The diff coverage is 16.32%.

@@              Coverage Diff              @@
##           features    #4443       +/-   ##
=============================================
- Coverage     95.87%   42.42%   -53.45%     
=============================================
  Files           111       92       -19     
  Lines         25222    20985     -4237     
  Branches       2474     2365      -109     
=============================================
- Hits          24181     8903    -15278     
- Misses          732    11480    +10748     
- Partials        309      602      +293

Flag	Coverage Δ
#docs	29.15% <12.24%> (-0.07%)	⬇️
#doctesting	29.15% <12.24%> (-0.07%)	⬇️
#linting	29.15% <12.24%> (-0.07%)	⬇️
#linux	42.42% <16.32%> (-53.29%)	⬇️
#nobyte	?
#numpy	40.81% <16.32%> (-52.56%)	⬇️
#pexpect	40.81% <16.32%> (-0.95%)	⬇️
#py27	40.81% <16.32%> (-53.14%)	⬇️
#py34	?
#py35	?
#py36	?
#py37	?
#trial	40.81% <16.32%> (-52.56%)	⬇️
#windows	?
#xdist	?

Impacted Files	Coverage Δ
testing/test_session.py	0% <0%> (-96.53%)	⬇️
testing/test_config.py	0% <0%> (-99.43%)	⬇️
testing/test_monkeypatch.py	0% <0%> (-99.62%)	⬇️
testing/test_recwarn.py	0% <0%> (-99.57%)	⬇️
testing/test_capture.py	0% <0%> (-99.24%)	⬇️
testing/test_pytester.py	0% <0%> (-87.46%)	⬇️
testing/test_runner.py	0% <0%> (-96.92%)	⬇️
testing/test_pluginmanager.py	0% <0%> (-99.59%)	⬇️
testing/test_parseopt.py	0% <0%> (-94.7%)	⬇️
src/_pytest/deprecated.py	100% <100%> (ø)	⬆️
... and 96 more

---

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[nicoddemus]

Nice, we just need to give the docs some love! 😁

[asottile]

I ran regendoc but decided against committing it since it twiddled all the python / python3.6 things, and broke for the new dataclasses assert. I'll make some (internal) tickets about this

[asottile]

@nicoddemus @RonnyPfannschmidt -- ready for another round of review :)

[nicoddemus]

Thanks @asottile! We just need to sort out the failure before we merge it. 👍

[asottile]

lol of course -- the test I just added 😆