buildman: Add python3-coverage

Add this package so we can run code-coverage tests for Binman.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/tools/buildman/requirements.txt b/tools/buildman/requirements.txt
index 4a31e69..564e548 100644
--- a/tools/buildman/requirements.txt
+++ b/tools/buildman/requirements.txt
@@ -1,3 +1,4 @@
+coverage==6.2
 jsonschema==4.17.3
 pyyaml==6.0
 yamllint==1.26.3