Fixed DeprecationWarning from using escape backslash in docstring

Used r"""text with backslashes""" to avoid deprecation warning that
looked like this when running pytest:

```
dpctl\__init__.py:64
  C:\devel\dpctl\dpctl\__init__.py:64: DeprecationWarning: invalid escape sequence \*
        """
```

Same could be seen on Linux.

Author: oleksandr-pavlyk

dpctl/__init__.py

@@ -56,7 +56,7 @@
 
 
 def get_include():
-    """
+    r"""
     Return the directory that contains the dpctl \*.h header files.
 
     Extension modules that need to be compiled against dpctl should use