add myself to CODEOWNERS for subprocess. (#3005)

diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index 550c956..cdb9fee 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -26,3 +26,6 @@
 **/*mime*                     @bitdancer
 **/*imap*                     @bitdancer
 **/*poplib*                   @bitdancer
+
+# subprocess
+**/*subprocess*               @gpshead