gnuradio: revert back to +python37 as the default variant; rev-bump

Closes: https://trac.macports.org/ticket/61173

1) GR-OOTs require the same variant, so they need to be switched that the same time as GR proper's variant is switched. My bad there!

2) Py38 has some changes to how processes are spawned that causes issues with some GR functionality. Leave Py38 as an option for users who need it and are not impacted by the issue, but keep the default at Py37 since it does not have this specific process spawning issue. Hope for a fix ...

science/gnuradio/Portfile

@@ -26,7 +26,7 @@ if {${subport} eq ${name}} {
     checksums       rmd160  0e3582fdca19c219e4f158f15bde4c943310fa97 \
                     sha256  c9aa8795f76075484dc50fcbad89988a39c91610c80d72182650b700a1529fd3 \
                     size    3381987
-    revision        1
+    revision        2
 
     long_description    ${description}: \
         This port is kept up with the GNU Radio release, \
@@ -35,7 +35,7 @@ if {${subport} eq ${name}} {
     conflicts           gnuradio37 gnuradio-next
 
     set python_versions { 3.6 3.7 3.8 }
-    set default_python_variant +python38
+    set default_python_variant +python37
 
     # temporary patch to fix and allow external setting for
     # CMAKE_C/CXX_STANDARD
@@ -123,7 +123,7 @@ subport gnuradio-next {
     checksums       rmd160  0a9e1294036e786116c96d68f9116e3f2798d325 \
                     sha256  bb2dab14e159a4b58ccf317ece3b4132959739f4b5b217d8e0add3c5ce581f5f \
                     size    3950832
-    revision        2
+    revision        3
 
     long_description    ${description}: \
         This port is kept up with the GNU Radio GIT 'master' \
@@ -140,7 +140,7 @@ subport gnuradio-next {
     conflicts           gnuradio gnuradio37
 
     set python_versions { 3.6 3.7 3.8 }
-    set default_python_variant +python38
+    set default_python_variant +python37
 
     # fix Quartz behaviour
     # TODO verify https://github.com/gnuradio/gnuradio/issues/2726