packaging: use "python3" for fedora29 ... and as buildrpm default

Change the buidrpm default. The CentOS targets explicitly use "python", and
changing the default will only influence Fedora 29.

A Python 3 package needs python3 dependencies, so pythonexe (and pythonver) is
used for specifying dependencies. Other OS versions will keep using "python" as
before ... or potentially change to explicit "python2". Fedora 29 packages can
thus also still be built for Python 2 - just not in the docker image that is
updated for Python 3.

contrib/packaging/buildrpm

@@ -6,7 +6,7 b''
 
 BUILD=1
 RPMBUILDDIR="$PWD/rpmbuild"
-PYTHONEXE=python2
+PYTHONEXE=python3
 
 while [ "$1" ]; do
     case "$1" in

contrib/packaging/docker/fedora29

@@ -7,8 +7,8 b' RUN dnf install -y \\'
 	gcc \
 	gettext \
 	make \
-	python-devel \
-	python-docutils \
+	python3-devel \
+	python3-docutils \
 	rpm-build
 
 # For creating repo meta data

contrib/packaging/mercurial.spec

@@ -2,7 +2,7 b''
 
 %define withpython %{nil}
 
-%global pythonexe python2
+%global pythonexe python3
 
 %if "%{?withpython}"
 
@@ -39,8 +39,8 b' BuildRequires: make, gcc, gettext'
 %if "%{?withpython}"
 BuildRequires: readline-devel, openssl-devel, ncurses-devel, zlib-devel, bzip2-devel
 %else
-BuildRequires: python >= %{pythonver}, python-devel, python-docutils >= 0.5
-Requires: python >= %{pythonver}
+BuildRequires: %{pythonexe} >= %{pythonver}, %{pythonexe}-devel, %{pythonexe}-docutils >= 0.5
+Requires: %{pythonexe} >= %{pythonver}
 %endif
 # The hgk extension uses the wish tcl interpreter, but we don't enforce it
 #Requires: tk