Bug 1874280 - use ConfigParser instead of SafeConfigParser for manifestupdate.py r=jgraham

Since our minimum supported Python version is 3.7, we can unconditionally
change to ConfigParser here.

Differential Revision: https://phabricator.services.mozilla.com/D198327

diff --git a/testing/web-platform/manifestupdate.py b/testing/web-platform/manifestupdate.py
--- a/testing/web-platform/manifestupdate.py
+++ b/testing/web-platform/manifestupdate.py
@@ -170,17 +170,17 @@ def ensure_manifest_directories(logger, 
                     raise
         elif not os.path.isdir(manifest_dir):
             raise IOError("Manifest directory is a file")
 
 
 def read_local_config(wpt_dir):
     src_config_path = os.path.join(wpt_dir, "wptrunner.ini")
 
-    parser = configparser.SafeConfigParser()
+    parser = configparser.ConfigParser()
     success = parser.read(src_config_path)
     assert src_config_path in success
     return parser
 
 
 def generate_config(logger, repo_root, wpt_dir, dest_path, force_rewrite=False):
     """Generate the local wptrunner.ini file to use locally"""
     if not os.path.exists(dest_path):