These are the last versions where keepalive has not been removed from the
development server with the protocol set to HTTP/1.1. TCP keepalive is required
for the socket communication in AgentManager.cs to work properly, since it only
opens one socket for the lifetime of the application.

Backslashes require another backslash for escaping; otherwise they are
interpreted in Python as escaping the following character, leading to unhelpful
error messages when the unity process exits.

Use Posix-style forward slashes in the Unity executable path on Windows, as
these won't ever be misinterpreted as being escape characters.