Fix broken CI

These changes should be revisited when we dropped support for nessai <0.7.0

10/10/22 - Merged in !1158 (closed), so this MR should fix the CI.

bilby/bilby_mcmc/proposals.py

@@ -547,6 +547,7 @@ class GMMProposal(DensityEstimateProposal):
     def _sample(self, nsamples=None):
         return np.squeeze(self.density.sample(n_samples=nsamples)[0])
 
+    @staticmethod
     def check_dependencies(warn=True):
         if importlib.util.find_spec("sklearn") is None:
             if warn:
@@ -734,6 +735,7 @@ class NormalizingFlowProposal(DensityEstimateProposal):
 
         return theta, float(log_factor)
 
+    @staticmethod
     def check_dependencies(warn=True):
         if importlib.util.find_spec("nflows") is None:
             if warn: