cmake: declare the pycsdiff target as a MODULE library

... to mark the intent that it is meant to be used by `dlopen` only.

Consequently, this forces the library to always use the `.so` suffix
which fixes the following test failure on macOS because Python ignores
shared libraries with the native `.dylib` suffix.
```
pycsdiff_py3......................................***Failed    0.02 sec
Traceback (most recent call last):
      File "<string>", line 1, in <module>
      ModuleNotFoundError: No module named 'pycsdiff'
```

src/CMakeLists.txt

@@ -105,7 +105,7 @@ macro(build_pycsdiff version)
     message(STATUS "Python ${version} binding enabled. "
         "The pycsdiff module will be built!")
 
-    add_library(pycsdiff_py${version} SHARED pycsdiff.cc)
+    add_library(pycsdiff_py${version} MODULE pycsdiff.cc)
     target_link_libraries(pycsdiff_py${version}
         PRIVATE ${Boost_PYTHON${PYTHON_VERSION_SUFFIX}_LIBRARY}
                 Python${version}::Module)