There is no need to support Python 2.7 anymore, Python 3.3+ has
`subprocess.DEVNULL`. This is good practice and also prevents file
handles from
staying open unnecessarily.
Also remove a couple unused or unneeded `__future__` imports.
Instead of maintaining separate swig interface files, we can use the API
headers directly. They implement the exact same C++ APIs and we can
conditionally include the python extensions as needed. To remove the
swig extensions from the API headers when building the LLDB
framework, we can use the unifdef tool when it is available. Otherwise
we just copy them as-is.
Differential Revision: https://reviews.llvm.org/D142926
