[docwriter] Drop support for Python < 3.5.

Python versions < 3.5 have reached end-of-life and as such, no
security or bug fixes will be provided for those versions.  See


for more information.

* 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.
10 files changed