Make setuptools license field SPDX-compliant

[pquentin]

https://spdx.org/licenses/
https://spdx.github.io/spdx-spec/SPDX-license-expressions/#d42-disjunctive-or-operator

As approved by @njsmith here: gitter.im/python-trio/general?at=6299a6fb4e38f759e29ffb3c

[codecov]

Codecov Report

Merging #26 (22387ba) into master (ebaa593) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #26   +/-   ##
=======================================
  Coverage   96.51%   96.51%           
=======================================
  Files           4        4           
  Lines          86       86           
  Branches       17       17           
=======================================
  Hits           83       83           
  Misses          2        2           
  Partials        1        1