Forbid changing frozen app properties

[asvetlov]

The following code was raising deprecation and forbidden now.

app.freeze()
app['foo'] = 'bar'

[kxepal]

What is the recommendation to do in this case: there is an app property, but it only could be initialized in async-way, so it has to be put in onstartup signal handler where app is frozen? Do some mutable placeholders?

[codecov-io]

Codecov Report

Merging #3948 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3948      +/-   ##
==========================================
+ Coverage   97.69%   97.91%   +0.22%     
==========================================
  Files          43       43              
  Lines        8761     9158     +397     
  Branches     1377     1546     +169     
==========================================
+ Hits         8559     8967     +408     
+ Misses         88       84       -4     
+ Partials      114      107       -7

Impacted Files	Coverage Δ
aiohttp/web_app.py	97.28% <100%> (ø)	⬆️
aiohttp/web_fileresponse.py	96.55% <0%> (-1.15%)	⬇️
aiohttp/helpers.py	97.53% <0%> (ø)	⬆️
aiohttp/web_protocol.py	92.35% <0%> (+0.61%)	⬆️
aiohttp/client_reqrep.py	97.67% <0%> (+0.68%)	⬆️
aiohttp/web_response.py	99.13% <0%> (+1.46%)	⬆️
aiohttp/web_request.py	98% <0%> (+1.82%)	⬆️

---

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[codecov-io]

Codecov Report

Merging #3948 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3948      +/-   ##
==========================================
+ Coverage   97.69%   97.91%   +0.22%     
==========================================
  Files          43       43              
  Lines        8761     9158     +397     
  Branches     1377     1546     +169     
==========================================
+ Hits         8559     8967     +408     
+ Misses         88       84       -4     
+ Partials      114      107       -7

Impacted Files	Coverage Δ
aiohttp/web_app.py	97.28% <100%> (ø)	⬆️
aiohttp/web_fileresponse.py	96.55% <0%> (-1.15%)	⬇️
aiohttp/helpers.py	97.53% <0%> (ø)	⬆️
aiohttp/web_protocol.py	92.35% <0%> (+0.61%)	⬆️
aiohttp/client_reqrep.py	97.67% <0%> (+0.68%)	⬆️
aiohttp/web_response.py	99.13% <0%> (+1.46%)	⬆️
aiohttp/web_request.py	98% <0%> (+1.82%)	⬆️

---

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 7642e4d...751adb8. Read the comment docs.