build(tox): pin linuxfabrik-lib>=3.1.0 so the test helpers are always available

The `lib.lftest` module in linuxfabrik-lib 3.1.0 ships the
`run()`, `attach_tests()`, `attach_each()`, `run_mariadb()` and
`MARIADB_LTS_IMAGES` helpers that the unit test files rely on.
Up to now tox installed `linuxfabrik-lib` without a version
constraint, so in theory CI could pull an older release from
PyPI in which those helpers do not exist yet. The unit tests
would then only work through the `sys.path.insert(0, '..')`
shadow that points Python at the monitoring-plugins/lib symlink,
which requires the lib repo to be checked out next to the
monitoring-plugins repo - a dev-machine convention, not
something a fresh CI runner guarantees.

Pin `linuxfabrik-lib>=3.1.0` in the tox environment so the
matrix always installs a lib that has the helpers, and the
sys.path shadow can stay pragmatic (local-iteration friendly)
without being load-bearing on CI.

Author: markuslf

tox.ini

@@ -9,7 +9,14 @@ no_package = true
 [testenv]
 deps =
     beautifulsoup4
-    linuxfabrik-lib
+    # Pinned to match requirements.txt so the tox matrix picks up the
+    # lftest helpers (run, attach_tests, attach_each, run_mariadb,
+    # MARIADB_LTS_IMAGES) that the unit tests rely on. Raise this on
+    # every lib bump that introduces new test helpers; otherwise tests
+    # that use them only work through the sys.path.insert(0, '..')
+    # shadow and break on any CI runner that does not have the lib
+    # repo checked out side-by-side.
+    linuxfabrik-lib>=3.1.0
     lxml
     psutil
     python-keystoneclient