mgr/dashboard: download setuptools_scm explicitly

otherwise we will fail to install the build dependencies of
`lazy-object-proxy` from the wheelhouse. as `lazy-object-proxy` does not
add `setuptools_scm` in its `setup.py`, instead it lists
`setuptools_scm` in `setup.cfg` and `pyproject.toml` as a `build-system`
requires. but unfortunately, `pip download` only downloads the
install/run-time dependencies at this moment. and `lazy-object-proxy`
does not offer binary package for at least python2.7.

ideally, `pip` should collects its dependencies like

Collecting setuptools_scm>=3.3.1 (from lazy-object-proxy->astroid<3,>=2.2.0->pylint->-r requirements-lint.txt (line 1))

so we have to list its build-time dependencies explicitly.

see also pypa/pip#6222

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/pybind/mgr/dashboard/requirements-lint.txt

@@ -7,3 +7,6 @@ flake8-colors
 #flake8-typing-imports; python_version >= '3'
 #pep8-naming
 rstcheck
+# see https://github.com/pypa/pip/issues/6222
+# pylint => astroid => lazy-object-proxy => setuptools_scm
+setuptools_scm