Set up code coverage

Rationale:
- tox should work the same as before
- coverage is too slow and disabled by default locally
- disable coverage on pypy and pypy3 (way too slow)
- add coverage info for pkg_resources
- make sure tests are ignored in coverage
- make sure all CI jobs are identifiable
- include coverage for xfail (too complicated otherwise)
- disable report on terminal

tests: Add regression test for #1081

This ensures we don't unknowingly change the behavior of this again.
Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: #1307

On pypy3 on Travis (version 5.8.0), running with coverage
enabled causes significant slowdown and test_build_meta
ends up timing out or otherwise failing. Until that is
fixed, skip coverage on pypy3 on Travis.

Some of these environment variables are necessary codecov to
detect that it is on travis/appveyor and thus doesn't need
an upload token.

Stop patching write_pkg_info

Fix unicode maintainer

Add support for maintainer in PKG-INFO