better deprecation warning for Response.body_as_unicode()

[kmike]

A follow-up to #4555.
In Scrapy master the following warning is logged:

[py.warnings] WARNING: /Users/kmike/envs/foo/lib/python3.8/site-packages/scrapy/http/response/text.py:66: ScrapyDeprecationWarning: Response.body_as_unicode() is deprecated, please use Response.text instead.
  warnings.warn('Response.body_as_unicode() is deprecated, '

It is not easily actionable because it doesn't point to the user code; it is unclear where the problem is actually happening. Adding stacklevel=2 fixes it:

[py.warnings] WARNING: /Users/kmike/svn/myspider/foo/bar/spiders/newsmonitor.py:53: ScrapyDeprecationWarning: Response.body_as_unicode() is deprecated, please use Response.text instead.
  print(response.body_as_unicode())

[codecov]

Codecov Report

Merging #4579 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4579      +/-   ##
==========================================
- Coverage   84.57%   84.55%   -0.03%     
==========================================
  Files         164      164              
  Lines        9931     9931              
  Branches     1477     1477              
==========================================
- Hits         8399     8397       -2     
- Misses       1266     1267       +1     
- Partials      266      267       +1     

Impacted Files	Coverage Δ
scrapy/http/response/text.py	100.00% <ø> (ø)
scrapy/core/downloader/__init__.py	89.47% <0.00%> (-1.51%)	⬇️

[elacuesta]

Nice catch!