aiohttp-session is not required

[oleksandr-kuzmenko]

Fixed this case:

$ pip install aiohttp-security
...
$ python3
>>> import aiohttp_security
...
ImportError: No module named 'aiohttp_session'

Another solution — throw error during SessionIdentityPolicy init

[asvetlov]

Sorry for inactivity but I prefer the second alternative.

[oleksandr-kuzmenko]

@asvetlov what do you think about this PR?

[asvetlov]

Please add # pragma: no cover for cases where aiohttp-session is not installed

[codecov]

Codecov Report

Merging #107 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   97.02%   97.03%   +0.01%     
==========================================
  Files          10       10              
  Lines         403      405       +2     
  Branches        8        8              
==========================================
+ Hits          391      393       +2     
  Misses         10       10              
  Partials        2        2

Impacted Files	Coverage Δ
aiohttp_security/session_identity.py	100% <100%> (ø)	⬆️

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[oleksandr-kuzmenko]

@asvetlov done

[jettify]

@asvetlov looks good to me, if you are ok with changes, I can merge

[asvetlov]

Thanks