Add myself to typing experts (#1288)

I cannot say that I know all about `typing` in Python, but I can still be helpful.

I don't want to be a codeowner on GitHub, because I will get too many notifications, but I can add myself here for better visibility for others: I am open to help, answer questions, and review PRs.

If @JelleZijlstra and @AlexWaygood agree :)

Author: sobolevn

core-developers/experts.rst

@@ -237,7 +237,7 @@ tracemalloc           vstinner
 tty                   Yhg1s*
 turtle                gregorlingl^, willingc
 types                 1st1
-typing                gvanrossum, JelleZijlstra*, AlexWaygood*, carljm
+typing                gvanrossum, JelleZijlstra*, AlexWaygood*, carljm, sobolevn*
 unicodedata           malemburg, ezio-melotti
 unittest              voidspace*, ezio-melotti, rbtcollins, gpshead
 unittest.mock         voidspace*