Refine requirements/dev.txt

[yan12125]

Issue #, if available:

Description of changes:

• py is a dependency of pytest, and apparently it's not imported directly in this project
• requests is used in examples only

Description of how you validated changes:

Checklist:

• Write/update tests

• make pr passes

• Update documentation

• Verify transformed template deploys and application functions as expected

• Add/update example to examples/2016-10-31

• This is a minor change so I skip these steps.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #998 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #998   +/-   ##
========================================
  Coverage    94.71%   94.71%           
========================================
  Files           69       69           
  Lines         3160     3160           
  Branches       606      606           
========================================
  Hits          2993     2993           
  Misses          85       85           
  Partials        82       82

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[yan12125]

Rebased for 1.12.0 release.