Move minimum_requirements.txt to separate directory (#352)

Dependabot struggles to get the correct lockfile to update when
both 'poetry.lock' and 'minimum_requirements.txt' are located in
the same directory and creates weird PR descriptions. See e.g.
#351, where
the actual base version is '0.42.1', but the PR mentions
'0.36.1' which is the version from 'minimum_requirements.txt'.

As a simple workaround, we move the 'minimum_requirements.txt' to
a separate directory.

tox.ini

@@ -45,7 +45,7 @@ setenv =
     PYTHONUNBUFFERED = yes
 commands =
     poetry install -E test
-    poetry run python -m pip install -r minimum_requirements.txt
+    poetry run python -m pip install -r .ci/minimum_requirements.txt
     poetry run pytest --reruns 1 --license-server={env:LICENSE_SERVER:} {env:PYTEST_MARKERS:} {posargs:-vv}
 
 [testenv:style]