On RHEL7 the python3 binary is actually naed python3.4.
The usual python3 is not available there.
Searching explicitly for python3.4 amongst the others will fix the issue.

Merged to upstream with 0.7.0

https://github.com/DinoTools/dionaea/pull/180
diff -ru dionaea-d2efb768e753a7f1ddca6dbf402548d741f33574.orig/configure.ac dionaea-d2efb768e753a7f1ddca6dbf402548d741f33574.new/configure.ac
--- dionaea-d2efb768e753a7f1ddca6dbf402548d741f33574.orig/configure.ac	2018-03-26 21:49:11.042568472 +0200
+++ dionaea-d2efb768e753a7f1ddca6dbf402548d741f33574.new/configure.ac	2018-03-26 21:49:29.854553159 +0200
@@ -580,7 +580,7 @@
 				      [cython_dir="$withval"],[cython_dir=$PATH])
 
 if test x$enable_cython = "xyes" ; then
-	AC_PATH_PROGS(CYTHON_BIN, [cython3 cython], "no", $cython_dir)
+	AC_PATH_PROGS(CYTHON_BIN, [cython3.4 cython3 cython], "no", $cython_dir)
 	if test "x$CYTHON_BIN" == "xno" ; then
 		enable_cython="no"
 	fi