binman: Add coverage tools info for Python 3

Test coverage with Python 3 requires a new package. Add details about
this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README b/tools/binman/README
index ac193f1..decca47 100644
--- a/tools/binman/README
+++ b/tools/binman/README
@@ -699,7 +699,7 @@
 
 To enable Python test coverage on Debian-type distributions (e.g. Ubuntu):
 
-   $ sudo apt-get install python-coverage python-pytest
+   $ sudo apt-get install python-coverage python3-coverage python-pytest
 
 
 Concurrent tests