Cast header to str in toxlsx

[arturponinski]

This PR has the objective of allowing non-str headers to be passed to toxlsx. This will align behavior with other methods, ie. todb, toavro...

Closes https://github.com/petl-developers/petl/issues/530

Additionaly this PR introduces a refactor of test_xlsx.py to utilize pytest fixtures

Changes

1. Added casting of header to str(text_type) in toxlsx and appendxlsx https://github.com/petl-developers/petl/pull/600/commits/c251bc5b7566a56a579c415f2b2a08be94767b77
2. Refactored xlsx test suite in a separate commit https://github.com/petl-developers/petl/pull/600/commits/15f85adf9ffebe360ca29700d67572e8c84fd8f6

Checklist

Use this checklist for assuring the quality of pull requests that include new code and or make changes to existing code.

• [ ] Source Code rules apply:
  • [x] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [x] All changes are documented in docs/changes.rst
• [ ] Versioning and history tracking rules apply:
  • [x] Using atomic commits when possible
  • [ ] Commits are reversible when possible
  • [x] There is no incomplete changes in the pull request
  • [x] There is no accidental garbage added in source code
• [ ] Testing rules apply:
  • [x] Tested locally using tox / pytest
  • [x] Automated testing passes (see CI)
  • [x] Unit test coverage has not decreased (see Coveralls)
• [ ] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [x] Ready to review
  • [ ] Ready to merge

[coveralls]

Coverage increased (+0.05%) to 91.813% when pulling e449b97ef6384de8f74067c0bfe80201d5fd6d7b on arturponinski:cast-header-to-str-in-toxlsx into a62aabcc3b20fe189a0875e405184533f1a72376 on petl-developers:master.

[juarezr]

• Do you think this is ready?

[arturponinski]

@juarezr I believe it is, merged :tada: