Search default python include folder for Eigen

[aulemahal]

Fixes #2. This uses sysconfig (builtin) to guess where python's own include files are installed and uses that folder a supplementary candidate to find Eigen's headers. This is useful for virtual environments.

This allows the installation of dependencies with conda and then of this package through pip (without having to pass the include folder of Eigen).

[yrobink]

Dear @aulemahal and @Zeitsperre,

Thank you for your PR, it is now in the master branch,