use tox-wheel 0.6 isolated_build flag

[graingert]

tox-wheel used to use its own flag to disable legacy builds, it
now uses the tox native config options:

ionelmc/tox-wheel@97a9e89

Contributor Checklist:

• I have updated the release notes at docs/source/NEWS.rst
• I have updated the automated tests.
• All tests pass on your local dev environment. See CONTRIBUTING.rst.

[codecov]

Codecov Report

Merging #217 (a0c39f6) into master (1f95713) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #217   +/-   ##
=======================================
  Coverage   83.29%   83.29%           
=======================================
  Files          87       87           
  Lines       11569    11569           
  Branches     1184     1184           
=======================================
  Hits         9636     9636           
  Misses       1815     1815           
  Partials      118      118           

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[adiroiban]

Thanks!