don't install lmxl when running '-nolxml' tests

in .travis/run.sh we call tox -e py-nolxml with the XML-related tests
to verify that they work despite lxml not being installed (using ElementTree
backend).
If we install lxml in requirements.txt unconditionally, then we are not
testing the absence of lxml...

requirements.txt

@@ -7,4 +7,3 @@ scipy==1.4.1; platform_python_implementation != "PyPy"
 munkres==1.1.2; platform_python_implementation == "PyPy"
 zopfli==0.1.6
 fs==2.4.11
-lxml==4.5.0

tox.ini

@@ -10,6 +10,7 @@ deps =
     pytest
     pytest-randomly
     -rrequirements.txt
+    !nolxml: lxml==4.5.0
 extras =
     ufo
     woff