Use sys.executable to run pip in the 'install' command.

[WarrenWeckesser]

When I install python from a source tarball with the standard ./configure script, there is no executable entry point called pip (it is called pip3).

This change modifies how pip is run in the install() function so that sys.executable is used to run the pip module.

[stefanv]

Thanks, @WarrenWeckesser!