Update lxml version (#4506)

ilevkivskyi · JukkaL · commit 7192a7578fc4 · 2018-01-24T17:02:57.000Z
The previous version was broken on Python 3.7 and broke our Travis build.
diff --git a/test-requirements.txt b/test-requirements.txt
@@ -1,7 +1,7 @@
 flake8
 flake8-bugbear; python_version >= '3.5'
 flake8-pyi; python_version >= '3.6'
-lxml==4.0.0
+lxml==4.1.1
 psutil==5.4.0
 pytest>=3.0
 pytest-xdist>=1.18
