makefile: use Python 3 by default when building docs as well
I copied the snippet defining `PYTHON` from the root Makefile.
Differential Revision: https://phab.mercurial-scm.org/D10251
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
end_of_line = lf
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true
end_of_line = lf
[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false
end_of_line = lf