remove scikit-learn dep and pin scikit-optimize>=0.8.1

Because //github.com/scikit-optimize/scikit-optimize/issues/931 is fixed.

Author: basnijholt

setup.py

@@ -56,8 +56,7 @@ def get_version_and_cmdclass(package_name):
         "distributed",
         "ipyparallel>=6.2.5",  # because of https://github.com/ipython/ipyparallel/issues/404
         "loky",
-        "scikit-optimize",
-        "scikit-learn<=0.23.1",  # because of https://github.com/scikit-optimize/scikit-optimize/issues/931, remove dep when fixed
+        "scikit-optimize>=0.8.1",  # because of https://github.com/scikit-optimize/scikit-optimize/issues/931
         "wexpect" if os.name == "nt" else "pexpect",
     ],
 }