packaging: introduce Python3 support as buildrpm --python3

Just overrule the HGPYTHON3 warning.

contrib/packaging/buildrpm

@@ -11,6 +11,7 b''
 
 BUILD=1
 RPMBUILDDIR="$PWD/rpmbuild"
+PYTHONEXE=python2
 
 while [ "$1" ]; do
     case "$1" in
@@ -18,6 +19,10 b' while [ "$1" ]; do'
         shift
         BUILD=
         ;;
+    --python3)
+        shift
+        PYTHONEXE=python3
+        ;;
     --withpython | --with-python)
         shift
         PYTHONVER=2.7.16
@@ -96,6 +101,7 b' rpmspec=$RPMBUILDDIR/SPECS/mercurial.spe'
 
 sed -e "s,^Version:.*,Version: $version," \
     -e "s,^Release:.*,Release: $release," \
+    -e "s/^%global pythonexe .*/%global pythonexe $PYTHONEXE/" \
     $specfile > $rpmspec
 
 echo >> $rpmspec

contrib/packaging/mercurial.spec

@@ -38,8 +38,8 b' BuildRequires: make, gcc, gettext'
 %if "%{?withpython}"
 BuildRequires: readline-devel, openssl-devel, ncurses-devel, zlib-devel, bzip2-devel
 %else
-BuildRequires: python >= 2.7, python-devel, python-docutils >= 0.5
-Requires: python >= 2.7
+BuildRequires: python >= %{pythonver}, python-devel, python-docutils >= 0.5
+Requires: python >= %{pythonver}
 %endif
 # The hgk extension uses the wish tcl interpreter, but we don't enforce it
 #Requires: tk
@@ -60,6 +60,8 b" sed -i '1c#! /usr/bin/env %{pythonexe}' "
 
 %build
 
+export HGPYTHON3=1
+
 %if "%{?withpython}"
 
 PYPATH=$PWD/%{pythonname}
@@ -91,6 +93,8 b" sed -i -e '1s|#!/usr/bin/env python$|#!/"
 %install
 rm -rf $RPM_BUILD_ROOT
 
+export HGPYTHON3=1
+
 %if "%{?withpython}"
 
 PYPATH=$PWD/%{pythonname}