certbot: fix dependencies

Fixes: https://trac.macports.org/ticket/58377

security/certbot/Portfile

@@ -20,7 +20,13 @@ checksums           rmd160  d1904c00d0cf7e54a70994ffb79798d148eb1ee6 \
                     sha256  d5203f32c50f3ec5a32df97e4affddbcd288a569678ecb5669adda21cd5ac3d9 \
                     size    342753
 
-python.versions     27 36 37
+if {[variant_isset python37]} {
+    python.default_version 37
+} elseif {[variant_isset python36]} {
+    python.default_version 36
+} else {
+    python.default_version 27
+}
 
 depends_lib         port:py${python.version}-acme \
                     port:py${python.version}-configargparse \
@@ -46,18 +52,12 @@ post-patch {
 }
 
 variant python27 conflicts python36 python37 description {Use Python 2.7} {
-    python.default_version 27
-
     depends_lib-append  port:py${python.version}-python2-pythondialog
 }
 
-variant python36 conflicts python27 python37 description {Use Python 3.6} {
-    python.default_version 36
-}
+variant python36 conflicts python27 python37 description {Use Python 3.6} {}
 
-variant python37 conflicts python27 python36 description {Use Python 3.7} {
-    python.default_version 37
-}
+variant python37 conflicts python27 python36 description {Use Python 3.7} {}
 
 if {![variant_isset python36] && ![variant_isset python37]} {
     default_variants +python27