Add flask as python dependency

Adding at request from Max F to allow experimentation with
making python UIs (how we actually implement our new
spline UI will vary).

Change-Id: I221880d43850ff3084994cf1910625e3568a4e87
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/tools/python/requirements.txt b/tools/python/requirements.txt
index 5d2234d..e7f23c3 100644
--- a/tools/python/requirements.txt
+++ b/tools/python/requirements.txt
@@ -37,3 +37,5 @@
 
 tensorflow
 tensorflow_datasets
+
+flask