python: provide a MagnumBindings::Python library for CMake superprojects.

The FindMagnumBindings so far worked for bindings as a subprojects due
to some weird magic, but as of 2bcc7b94d3
it no longer does, which is how it should be as no such target was
created by the subproject buildsystem at all until now.

src/python/CMakeLists.txt

@@ -86,3 +86,10 @@ configure_file(${CMAKE_CURRENT_SOURCE_DIR}/setup.py.cmake
                ${CMAKE_CURRENT_BINARY_DIR}/setup.py.in)
 file(GENERATE OUTPUT ${output_dir}/setup.py
     INPUT ${CMAKE_CURRENT_BINARY_DIR}/setup.py.in)
+
+# MagnumPythonBindings library and alias, just for superprojects to have
+# something to link to get to the headers
+add_library(MagnumPythonBindings INTERFACE)
+set_target_properties(MagnumPythonBindings PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES ${PROJECT_SOURCE_DIR}/src)
+add_library(MagnumBindings::Python ALIAS MagnumPythonBindings)