check version bump on library modifications (#33)

tox.ini

@@ -68,9 +68,14 @@ deps =
 commands =
   charm: mypy {[vars]src_path} {posargs}
   lib: mypy --python-version 3.8 {[vars]lib_path} {posargs}
+  lib: /usr/bin/env sh -c 'for m in $(git diff main --name-only {[vars]lib_path}); do \
+  lib:    if ! git diff $m | grep -q "+LIBPATCH\|+LIBAPI"; then \
+  lib:        echo "You forgot to bump the version on $m!"; exit 1; \
+  lib:    fi; done'
   unit: mypy {[vars]tst_path}/unit {posargs}
   integration: mypy {[vars]tst_path}/integration {posargs} \
     --exclude {[vars]tst_path}/integration/ingress-requirer-mock
+allowlist_externals = /usr/bin/env
 
 [testenv:unit]
 description = Run unit tests