[FIX] Default to using scipy v1.8.0+ API (#646)

* As most users will be installing the latest scipy release when they install
  POT, default to using the scipy v1.8.0+ API and catch the ModuleNotFoundError in
  the rare case in which scipy v1.7.3 or older is required, such as Python 3.7.
* Always import from the scipy.optimize.{_}linesearch API as the newest version
  of scipy that supports Python 3.7 is v1.7.3 which does not support
  'from scipy.optimize import scalar_search_armijo', just as scipy v1.14.0+ does
  not.

Author: matthewfeickert

ot/optim.py

@@ -17,9 +17,10 @@
 with warnings.catch_warnings():
     warnings.simplefilter("ignore")
     try:
-        from scipy.optimize import scalar_search_armijo
-    except ImportError:
         from scipy.optimize._linesearch import scalar_search_armijo
+    except ModuleNotFoundError:
+        # scipy<1.8.0
+        from scipy.optimize.linesearch import scalar_search_armijo
 
 # The corresponding scipy function does not work for matrices