Enable Packit pull-request CI

Enable it on all currently supported Fedora/EPEL architectures.

Packit doesn't support our way of preparing source RPMs via the command
'tito build --srpm --test', but creates it's own source RPM instead.
This doesn't seem to cause a practical problems, so we are good to go
for now till the Packit RFE is implemented:
https://github.com/packit/packit-service/issues/920

Fixes: #395

.packit.yml

@@ -0,0 +1,9 @@
+---
+jobs:
+  - job: copr_build
+    trigger: pull_request
+    metadata:
+      targets:
+        - fedora-all
+        - epel-7-x86_64
+        - epel-8-x86_64