Fix python 36 issue

Author: perseoGI

conan/tools/cmake/toolchain/blocks.py

@@ -1027,7 +1027,8 @@ def get_exact_type(self, key, value):
                 if not var_type:
                     raise ConanException(f'CMakeToolchain needs type defined for cache variable "{key}"')
                 if var_type not in self.CMAKE_CACHE_TYPES:
-                    raise ConanException(f'CMakeToolchain invalid type "{var_type}" for cache variable "{key}". Possible types: {', '.join(self.CMAKE_CACHE_TYPES)}')
+                    possible_types = ', '.join(self.CMAKE_CACHE_TYPES)
+                    raise ConanException(f'CMakeToolchain invalid type "{var_type}" for cache variable "{key}". Possible types: {possible_types}')
                 docstring = value.get("docstring")
                 if not docstring:
                     raise ConanException(f'CMakeToolchain needs docstring defined for cache variable "{key}"')