tox.ini: use coverage.py instead of pytest-cov; run in --parallel-mode; use codecov instead of coveralls

We no longer use coverage.py through the pytest-cov plugin, as it does not
support --parallel-mode (or at least I wasn't able to figure it out).
We run pytest through `coverage run -m pytest` instead, which does the same
as `pytest --cov`.

Running `tox` locally by default will now run pytest and coverage on 2.7, 3.5 and 3.6; it will then
combine coverage data from all three, and finally generate a coverage report in htmlcov/index.html

We need tp temporarily install coverage.py from the mercurial repo instead of the latest release
because we require a patch that is not available yet with coverage 4.2:
https://bitbucket.org/ned/coveragepy/pull-requests/118/check-source-isdir-not-just-exists-there/diff

Finally, I replaced Coveralls with Codecov, as the latter is better designed and also supports Appveyor.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py35, py36
+envlist = py{27,35,36}-cov, htmlcov
 skip_missing_interpreters = true
 
 [testenv]
@@ -10,33 +10,38 @@ basepython =
     py35: {env:TOXPYTHON:python3.5}
     py36: {env:TOXPYTHON:python3.6}
 deps =
+    # we install coverage from source until 4.3 is released, because of this:
+    # https://bitbucket.org/ned/coveragepy/pull-requests/118/check-source-isdir-not-just-exists-there/diff
+    cov: hg+https://bitbucket.org/ned/coveragepy#egg=coverage
     pytest
     -rrequirements.txt
 install_command =
-    {envpython} -m pip install -v {opts} {packages}
+    pip install -v {opts} {packages}
 commands =
-    # run the test suite against the package installed inside tox env
-    py.test {posargs:--pyargs fontTools}
+    # run the test suite against the package installed inside tox env.
+    # We use parallel mode and then combine later so that coverage.py will take
+    # paths like .tox/py36/lib/python3.6/site-packages/fontTools and collapse
+    # them into Lib/fontTools.
+    cov: coverage run --parallel-mode -m pytest {posargs:--pyargs fontTools}
+    nocov: pytest {posargs:--pyargs fontTools}
 
-[testenv:coverage]
+[testenv:htmlcov]
 basepython = {env:TOXPYTHON:python3.5}
 deps =
-    {[testenv]deps}
-    pytest-cov
+    hg+https://bitbucket.org/ned/coveragepy#egg=coverage
 skip_install = true
 commands =
-    # measure test coverage and create html report
-    py.test --cov --cov-report html {posargs}
+    coverage combine
+    coverage html
 
-[testenv:coveralls]
-passenv = TRAVIS TRAVIS_JOB_ID TRAVIS_BRANCH
-basepython=python3.5
+[testenv:codecov]
+passenv = *
+basepython = {env:TOXPYTHON:python}
 deps =
-    {[testenv:coverage]deps}
-    coveralls
+    hg+https://bitbucket.org/ned/coveragepy#egg=coverage
+    codecov
 skip_install = true
 ignore_outcome = true
 commands =
-    # measure test coverage and upload report to coveralls
-    py.test --cov
-    coveralls
+    coverage combine
+    codecov --env TOXENV