Bug Fixes

• Use a proper type for the thread indent (fixes 32-bit build for 3.7.)
  (by @1st1 in 700582a for #172)

• Fix cancellation race in loop.sock_recv() and loop.sock_recv_into()
  methods. (by @andr-04 and @1st1 in 298851b for #169)

• Sync SSL error messages with CPython's SSL implementation.
  (by @1st1 in c3aeff2)

• Fix SSLTransport.abort() to mark the transport as closed.
  (by @1st1 in ba25d8b)

• Detect if libuv submodule has not been checked out in setup.py.
  (by @1st1 in dd8060d)