replace usage of sys.stdout.encoding with sys.getdefaultencoding()

In some environments, `sys.stdout.encoding` may return `None`. 

This is encountered when knitting an rmarkdown document using the reticulate engine. In that context, `sys.stdout` is replaced with a `StringIO()` object to capture all output. `StringIO().encoding` returns `None`

closes rstudio/reticulate#1508

Author: t-kalinowski

pytensor/link/c/cmodule.py

@@ -2743,7 +2743,7 @@ def get_cxx_library_dirs():
 
         maybe_lib_dirs = [
             [pathlib.Path(p).resolve() for p in line[len("libraries: =") :].split(":")]
-            for line in stdout.decode(sys.stdout.encoding).splitlines()
+            for line in stdout.decode(sys.getdefaultencoding()).splitlines()
             if line.startswith("libraries: =")
         ]
         if len(maybe_lib_dirs) > 0: