Create socket file in /tmp

[myjian]

When the mypy daemon runs in Windows Subsystem for Linux (WSL) under a DriveFs directory, it would fail to bind to the socket file.

From mypy/dmypy_server.py:

sock = socket.socket(socket.AF_UNIX)
sock.bind(self.sockname)  # <-- stuck here
sock.listen(1)

This pull request changes the path to /tmp, so on Windows it will be always created under the VolFs, which provides full support for Linux file system features.

[msullivan]

Just moving the socket to /tmp/dmypy.sock means that mypy-daemon can only be run on one codebase at a time per system. We need to generate a temporary directory using tempfile and put it there.

[msullivan]

Thanks!