Clean build folder in pep517 builds

[mcarans]

Cleans the {project_root}/build dir for pep517 builds similarly to legacy builds. Test added for this.

Fixes #15

Doing a fresh checkout of the repository without any of my changes and running tox, some tests for Python 3.x fail.

I think the reason is because of what is output for PEP517 runs. For example, for Py2.7 and Py3.5, the test_enabled_pep517 test outputs 'Building wheel for foobar (PEP 517)'. However, for Py3.6+, it outputs 'Building wheel for foobar (pyproject.toml)' I have updated the tests to fix this.

[codecov]

Codecov Report

Merging #16 (d99b035) into master (7e7f451) will increase coverage by 1.58%.
The diff coverage is 100.00%.

❗ Current head d99b035 differs from pull request most recent head 81f0e4c. Consider uploading reports for the commit 81f0e4c to get more accurate results

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   81.92%   83.50%   +1.58%     
==========================================
  Files           3        3              
  Lines         177      194      +17     
  Branches       14       18       +4     
==========================================
+ Hits          145      162      +17     
  Misses         28       28              
  Partials        4        4              

Impacted Files	Coverage Δ
src/tox_wheel/plugin.py	67.01% <100.00%> (+1.05%)	⬆️
tests/test_tox_wheel.py	100.00% <100.00%> (ø)

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[ionelmc]

Looks good, please also add yourself to AUTHORS.rst

Is the py3.6 check only needed to support 2.7? It would appear that I need to switch to github actions but the 2.7 support there is pretty bad so it would be time to also drop support for 2.7

[ionelmc]

Alright thank you.