.gitignore: Ignore any html coverage directory

This is created when checking code coverage of Python tools. Ignore it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/.gitignore b/.gitignore
index e66aa86..35034de 100644
--- a/.gitignore
+++ b/.gitignore
@@ -95,3 +95,6 @@
 
 # Python cache
 __pycache__
+
+# Python code coverage output (python3-coverage html)
+/htmlcov/