test-requirements.txt: do not install lxml for py38

Author: blueyed

test-requirements.txt

@@ -2,7 +2,8 @@ attrs>=18.0
 flake8
 flake8-bugbear; python_version >= '3.5'
 flake8-pyi; python_version >= '3.6'
-lxml==4.2.4
+# Temporarily disabled, until fixed for py38.
+lxml==4.2.4; python_version < '3.8'
 mypy_extensions>=0.4.0,<0.5.0
 psutil>=4.0
 pytest>=4.4