Fix string comparison to be sh compatible

from `bash -c 'help test'`:

```
string1 == string2
       True if the strings are equal.  = may be used in place of == for
strict POSIX compliance.
```

sbin/exabgp

@@ -20,7 +20,7 @@ for INTERPRETER in "$INTERPRETER" python3 pypy3; do
     INTERPRETER="$(command -v "$INTERPRETER")" && break
 done
 
-if [ "$INTERPRETER" == "" ]; then
+if [ "$INTERPRETER" = "" ]; then
 	echo "ExaBGP could not find a python3 interpreter"
 	exit 1
 fi