Hello,

Here are some comments about the patches:

1) poplib_01_socket_shutdown_v3.diff: looks fine to me

2) poplib_02_server_capabilities_v3.diff:

- please try to follow PEP 8 (i.e. `capa = {}` not `capa={}`)
- I think the capa() result should be a dict mapping str keys to str values (not bytes), since that part of the POP3 protocol seems to have a well-defined character set (ASCII)

3) poplib_03_starttls_v3.diff:

- same comment about PEP 8
- why did you change the signature of the _create_socket() method? it looks gratuitous
- the new method should be called starttls() as in other modules, not stls()
- starttls() should only take a context argument; no need to support separate keyfile and certfile arguments
- what is the point of catching errors like this:

+        except error_proto as _err:
+            resp = _err