makefile: build on CentOS 6 with a Python 2.7

Differential Revision: https://phab.mercurial-scm.org/D189

Makefile

@@ -276,13 +276,13 b' docker-centos5:'
 
 centos6:
 	mkdir -p packages/centos6
-	contrib/buildrpm
+	contrib/buildrpm --withpython
 	cp rpmbuild/RPMS/*/* packages/centos6
 	cp rpmbuild/SRPMS/* packages/centos6
 
 docker-centos6:
 	mkdir -p packages/centos6
-	contrib/dockerrpm centos6
+	contrib/dockerrpm centos6 --withpython
 
 centos7:
 	mkdir -p packages/centos7

contrib/docker/centos6

@@ -7,3 +7,5 b' RUN yum install -y gettext'
 RUN yum install -y tar
 # For creating repo meta data
 RUN yum install -y createrepo
+# For python
+RUN yum install -y readline-devel openssl-devel ncurses-devel zlib-devel bzip2-devel