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
--- a/doc/Makefile Mon Mar 22 13:14:11 2021 -0700
+++ b/doc/Makefile Mon Mar 22 13:21:12 2021 -0700
@@ -6,7 +6,14 @@
PREFIX=/usr/local
MANDIR=$(PREFIX)/share/man
INSTALL=install -m 644
-PYTHON?=python
+# Default to Python 3.
+#
+# Windows ships Python 3 as `python.exe`, which may not be on PATH. py.exe is.
+ifeq ($(OS),Windows_NT)
+PYTHON?=py -3
+else
+PYTHON?=python3
+endif
RSTARGS=
export HGENCODING=UTF-8