[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
  • New functions have docstrings with examples that can be run with doctest (no new functions)
  • New functions are included in API docs (no new functions)
  • Docstrings include notes for any changes to API or behaviour (no changes in API behavior)
  • All changes documented in docs/changes.rst
• Testing
  • (Optional) Tested local against remote servers
  • Travis CI passes (unit tests run under Linux)
  • AppVeyor CI passes (unit tests run under Windows)
  • Unit test coverage has not decreased (see Coveralls)
• Changes
  • (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 d120c68 on MihanixA:poc-typing into 84c7b7f on petl-developers:master.

[juarezr]

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