#5713: Handle 421 error codes during sendmail by closing the socket.

This is a partial fix to the issue of servers disconnecting unexpectedly; in
this case the 421 says they are disconnecting, so we close the socket and
return the 421 in the appropriate error context.

Original patch by Mark Sapiro, updated by Kushal Das, with additional
tests by me.
remove the long obsolete mention of universal newlines mode only being
available when configured at compile time.
#5713: fix timing issue in smtplib tests.
#5712: Preemptively fix some other possible timing issues.
- Issue #16754: Fix the incorrect shared library extension on linux. Introduce
  two makefile macros SHLIB_SUFFIX and EXT_SUFFIX. SO now has the value of
  SHLIB_SUFFIX again (as in 2.x and 3.1). The SO macro is removed in 3.4.