Adjust Windows DLL search path for Python 3.8+ (#1440)

Fixes #1439

Required by python/cpython#80266

python/MaterialX/__init__.py

@@ -1,3 +1,24 @@
+# Python 3.8+ on Windows: DLL search paths for dependent
+# shared libraries
+# Refs.:
+# - https://github.com/python/cpython/issues/80266
+# - https://docs.python.org/3.8/library/os.html#os.add_dll_directory
+import os
+import sys
+if sys.platform == "win32" and sys.version_info >= (3, 8):
+    import importlib.metadata
+    try:
+        importlib.metadata.version('MaterialX')
+    except importlib.metadata.PackageNotFoundError:
+        # On a non-pip installation, this file is in %INSTALLDIR%\python\MaterialX
+        # We need to add %INSTALLDIR%\bin to the DLL path.
+        mxdir = os.path.dirname(__file__)
+        pydir = os.path.split(mxdir)[0]
+        installdir = os.path.split(pydir)[0]
+        bindir = os.path.join(installdir, "bin")
+        if os.path.exists(bindir):
+            os.add_dll_directory(bindir)
+
 from .main import *
 from .colorspace import *