Use `ez_setup.py` and `get-pip.py` to setup pip

Modified `build_package_ensurepip` to support the Python versions which don't have `ensurepip` module, with using https://bootstrap.pypa.io/ez_setup.py and https://bootstrap.pypa.io/get-pip.py. Actually, at the time of writing this, https://bootstrap.pypa.io/ez_setup.py is still pointed to problematic setuptools 4.0.1 (#190, #191, #192), you need to set `EZ_SETUP_URL=https://bitbucket.org/pypa/setuptools/raw/5.0.1/ez_setup.py` to use this patch for now.

I believe that this also solves #173 partially.