Python versions < 3.5 have reached end-of-life and as such, no
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* Jamfile (RefDoc): Add `site' parameter.
* builds/detect.mk (std_setup): Update Python version requirement.
* builds/freetype.mk (refdoc-venv): Use pip as `python -m pip'.
* builds/unix/ax_compare_version.m4,
builds/unix/ax_prog_python_version.m4: Macros to detect Python
version. New files.
* builds/unix/configure.raw: Check for Python >= 3.5 and remove
check for `pip'.
* docs/CHANGES, docs/INSTALL.GNU, docs/README: Updated.
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