tox: add extras; allow to test with/without lxml

calling 'tox -e py37-cov-nolxml' will run tests without lxml,
using built-in ElementTree library

tox.ini

@@ -1,19 +1,20 @@
 [tox]
 minversion = 3.0
-envlist = py{27,36,37}-cov, htmlcov
+envlist = py{27,37}-cov, htmlcov
 
 [testenv]
 deps =
     cov: coverage>=4.3
     pytest
     -rrequirements.txt
-install_command =
-    pip install -v {opts} {packages}
+extras =
+    ufo
+    woff
+    unicode
+    interpolatable
+    !nolxml: lxml
 commands =
-    # 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.
+    # test with or without coverage, passing extra positonal args to pytest
     cov: coverage run --parallel-mode -m pytest {posargs}
     nocov: pytest {posargs}