[POC] added typing to materialize (breaks py27 tests)

[MihanixA]

Proof of concept: typing for petl

Changes

1. Added typing for some functions in materialize.py

Checklist

Checklist for for pull requests including new code and/or changes to existing code...

• [ ] Code
  • [ ] Includes unit tests
  • [x] New functions have docstrings with examples that can be run with doctest (no new functions)
  • [x] New functions are included in API docs (no new functions)
  • [x] Docstrings include notes for any changes to API or behaviour (no changes in API behavior)
  • [ ] All changes documented in docs/changes.rst
• [ ] Testing
  • [x] (Optional) Tested local against remote servers
  • [ ] Travis CI passes (unit tests run under Linux)
  • [ ] AppVeyor CI passes (unit tests run under Windows)
  • [x] Unit test coverage has not decreased (see Coveralls)
• [ ] Changes
  • [x] (Optional) Just a proof of concept
  • [ ] (Optional) Work in progress
  • [ ] Ready to review
  • [ ] Ready to merge

[coveralls]

Coverage increased (+0.0006%) to 91.313% when pulling d120c6845c4f82ce684c50583f7a905047b8ff82 on MihanixA:poc-typing into 84c7b7fe6e46dc676c229be43720912144dd50e6 on petl-developers:master.

[juarezr]

PR #563 could drop python2.7, so referencing it here.