Add tool for creating a public manifest based on an internal one

Make the public manifest creation as easy as possible: Have a tool that
does all required modifications on the internal manifest to generate the
public "Fairphone Open" manifest.

Issue: FP2N-362
Issue: INFRA-206
Change-Id: I33a18442db8b0436d4423191b3de2b4dfb7dd64f
diff --git a/requirements-dev.txt b/requirements-dev.txt
index be90c87..83dc34e 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -3,6 +3,7 @@
 flake8 ~= 3.6
 flake8-colors
 flake8-docstrings
+lxml ~= 4.4.1
 mypy ~= 0.701
 pep8-naming
 pylint ~= 2.1