Allow providing an override env in Request.verify

[mmeisinger]

Enable specifying an explicit override environment "env" in the Request.verify() and call in order to avoid a pop from the environment stack. This makes cases with thread concurrency deterministic and avoids the Environment.current() stack access.

Example:

import itunesiap
response = itunesiap.verify(receipt, apple_shared_secret, env=itunesiap.env.production)

[codecov-io]

Codecov Report

Merging #35 into master will increase coverage by 0.07%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage    89.3%   89.36%   +0.07%     
==========================================
  Files           8        8              
  Lines         327      329       +2     
==========================================
+ Hits          292      294       +2     
  Misses         35       35

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[coveralls]

Coverage increased (+0.07%) to 89.362% when pulling 910ec00 on mmeisinger:override_env into 81f31e7 on youknowone:master.

[coveralls]

Coverage increased (+0.07%) to 89.362% when pulling 910ec00 on mmeisinger:override_env into 81f31e7 on youknowone:master.

[coveralls]

Coverage increased (+0.07%) to 89.362% when pulling 910ec00 on mmeisinger:override_env into 81f31e7 on youknowone:master.

[coveralls]

Coverage increased (+0.07%) to 89.362% when pulling 910ec00 on mmeisinger:override_env into 81f31e7 on youknowone:master.

[youknowone]

Thanks, this is a nice fix for the mis-designed env api.

[youknowone]

The global environment stack is deprecated now.
Now this is the major way to run itunesiap.