Update lxml from 4.5.1 to 4.5.2

requirements.txt

@@ -4,7 +4,7 @@ contextlib2==0.6.0.post1
 future==0.18.2
 inflection==0.5.0
 isodate==0.6.0
-lxml==4.5.1
+lxml==4.5.2
 markdown==3.1.1 # pyup: >=3.1,<3.2 # 3.2 drops Python 2.7 support
 Pillow==7.2.0
 python-redmine==2.3.0