Update CODEOWNERS (#17356)

Add Ethan Furman for enum, cgi, and cgitb.
diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index 32d2114..7b415ac 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -114,6 +114,9 @@
 **/*ftplib                    @giampaolo
 **/*shutil                    @giampaolo
 
+**/*enum*                     @ethanfurman
+**/*cgi*                      @ethanfurman
+
 # macOS
 /Mac/                         @python/macos-team
 **/*osx_support*              @python/macos-team