Prefer Python 3 tools

The last three commits already made configure recommend installing
Python 3 packages and look for versioned executables, however with a low
priority.

This is a problem on systems such as OpenBSD 6.5 with a default Python
version at 2.7, where 3.7 flavored Python packages get installed with
a "-3" binary suffix.  That is, when both rst2man and rst2man-3 are
installed at configure time, the lower version will be picked unless
explicitly passed through `--with-feature' arguments.

Regardless of this specific case, trying more specificly versioned tool
names first seems correctly in line with recent development and less
error prone, so change it accordingly.

configure.ac

@@ -41,7 +41,7 @@ AC_ARG_WITH([rst2man],
   AS_HELP_STRING([--with-rst2man=PATH], [Location of rst2man (auto)]),
   [RST2MAN="$withval"],
   AC_CHECK_PROGS(RST2MAN,
-    [rst2man rst2man.py rst2man-3.6 rst2man-3],
+    [rst2man-3.6 rst2man-3 rst2man rst2man.py],
     [no]))
 if test "x$RST2MAN" = "xno"; then
   AC_MSG_ERROR(
@@ -52,7 +52,7 @@ AC_ARG_WITH([sphinx-build],
   AS_HELP_STRING([--with-sphinx-build=PATH], [Location of sphinx-build (auto)]),
   [SPHINX="$withval"],
   AC_CHECK_PROGS(SPHINX,
-    [sphinx-build sphinx-build-3.6 sphinx-build-3],
+    [sphinx-build-3.6 sphinx-build-3 sphinx-build],
     [no]))
 if test "x$SPHINX" = "xno"; then
   AC_MSG_ERROR(
@@ -63,7 +63,7 @@ AC_ARG_WITH([rst2html],
   AS_HELP_STRING([--with-rst2html=PATH], [Location of rst2html (auto)]),
   [RST2HTML="$withval"],
   AC_CHECK_PROGS(RST2HTML,
-     [rst2html rst2html.py rst2html-3.6 rst2html-3],
+     [rst2html-3.6 rst2html-3 rst2html rst2html.py],
      "no"))
 
 if test "x$RST2HTML" = "xno"; then