rpm.spec: bump python dependency to 2.6

We are about to drop 2.4 requirement in Mercurial's setup.py, we bump rpm
dependency first for the sake of smaller changeset. Clean up of the spec file
can come after the dependency is actually dropped.

contrib/mercurial.spec

@@ -37,8 +37,8 b' BuildRequires: make, gcc, gettext'
 %if "%{?withpython}"
 BuildRequires: readline-devel, openssl-devel, ncurses-devel, zlib-devel, bzip2-devel
 %else
-BuildRequires: python >= 2.4, python-devel, python-docutils >= 0.5
-Requires: python >= 2.4
+BuildRequires: python >= 2.6, python-devel, python-docutils >= 0.5
+Requires: python >= 2.6
 %endif
 # The hgk extension uses the wish tcl interpreter, but we don't enforce it
 #Requires: tk