slapos-node package: better use of python3.7

This is a fixup of 99d9c776

This way of forcing python3.7 is better because the name of the part is
still "python3" hence in the package we have

/opt/slapos/parts/python3/bin/python3

instead of

/opt/slapos/parts/python3.7/bin/python3

component/slapos/obs.cfg

@@ -31,7 +31,6 @@ init +=
   assert buildout['directory'] == buildout['destdir'] + buildout['rootdir'], (
     "Buildout MUST BE launched in destdir/rootdir (currently launched in %s but should be launched in %s)" %
     (buildout['directory'], buildout['destdir'] + buildout['rootdir']))
-part = python3.7
 
 [python3-common]
 configure-options +=
@@ -39,6 +38,10 @@ configure-options +=
 environment +=
   DESTDIR=${buildout:destdir}
 
+# Force python3.7 for a while to be compatible with more SR
+[python3]
+<= python3.7
+
 [bison]
 configure-options +=
   --prefix=${buildout:rootdir}/parts/${:_buildout_section_name_}