Add self and idlelib to CODEOWNERS file. (#3889)

Followed format of other entries.
diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index 67c03a8..8e0647f 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -49,3 +49,5 @@
 **/*heapq*                    @rhettinger
 **/*functools*                @ncoghlan @rhettinger
 **/*decimal*                  @rhettinger @skrah
+
+**/*idlelib*                  @terryjreedy