Deprecate the scrapy.downloadermiddlewares.decompression module

[Laerte]

Closes #5546

[Gallaecio]

We have a policy not to remove features right away, and instead issue deprecation warnings for at least 1 year.

So the change at this stage should consist of doing this in the module:

from warnings import warn

from scrapy.exceptions import ScrapyDeprecationWarning

warn(
    'scrapy.downloadermiddlewares.decompression is deprecated',
    ScrapyDeprecationWarning,
    stacklevel=2,
)

The tests can remain there until the feature is removed.

[codecov]

Codecov Report

Merging #5547 (fe08a11) into master (b51b52f) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5547      +/-   ##
==========================================
- Coverage   88.74%   88.73%   -0.02%     
==========================================
  Files         162      162              
  Lines       10771    10774       +3     
  Branches     1843     1843              
==========================================
+ Hits         9559     9560       +1     
- Misses        939      940       +1     
- Partials      273      274       +1     

Impacted Files	Coverage Δ
scrapy/downloadermiddlewares/decompression.py	100.00% <100.00%> (ø)
scrapy/core/downloader/__init__.py	90.97% <0.00%> (-1.51%)	⬇️

[Laerte]

@Gallaecio Like this?

[Gallaecio]

Yes, thank you!

[kmike]

Looks good, thanks!