bionic-amd64-tf-m-build: add sphinx tabs support

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I92cad0caeba6668d975d3b0cabd9cfd422c861df
diff --git a/bionic-amd64-tf-m-build/requirements_python3.txt b/bionic-amd64-tf-m-build/requirements_python3.txt
index 03a1b20..4f43ff1 100644
--- a/bionic-amd64-tf-m-build/requirements_python3.txt
+++ b/bionic-amd64-tf-m-build/requirements_python3.txt
@@ -8,6 +8,7 @@
 m2r==0.2.1
 Sphinx==2.0.1
 sphinx-rtd-theme==0.4.3
+sphinx-tabs==1.1.13
 sphinxcontrib-plantuml==0.15
 sphinxcontrib-svg2pdfconverter==1.1.1
 xmltodict==0.12.0