Add license to project metadata

[cdce8p]

What do these changes do?

Add the SPDX identifier for the BSD license to the project metadata.
https://github.com/aio-libs/aiocache/blob/master/LICENSE

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.76%. Comparing base (fe28c47) to head (86fc44d).
Report is 86 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #917   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files          36       36           
  Lines        3794     3796    +2     
=======================================
+ Hits         3785     3787    +2     
  Misses          9        9           

Flag	Coverage Δ
unit	99.76% <ø> (+<0.01%)	⬆️

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