MT#56773 debian: support python3

The python binary is gone, and use and support python3 instead.

Fixes:

| Setting up repoapi (0.4.6+0~20230705093550.1012+bookworm~1.gbp7c23af) ...
| /var/lib/dpkg/info/repoapi.postinst: 21: python: not found
| dpkg: error processing package repoapi (--configure):
|  installed repoapi package post-installation script subprocess returned error exit status 127

Change-Id: I54c02826da75fb2a01ee307aeab850978034c9da

debian/repoapi.postinst

@@ -18,7 +18,7 @@ LIB_DIR=/var/lib/repoapi
 # the debian-policy package
 
 mk_dj_secret() {
-  python -c "import random,string;print '%s'%''.join([random.SystemRandom().choice(\"{}{}{}\".format(string.ascii_letters, string.digits, string.punctuation)) for i in range(63)])" ;
+  python3 -c "import random, string; print(''.join(random.SystemRandom().choice(string.ascii_letters + string.digits + string.punctuation) for _ in range(63)))";
 }
 
 case "$1" in