Replace use of imp with importlib.util

imp is deprecated and will go away in Python 3.12 (which is very
soon).

This implementation based on the importlib docs [1] and passes
the quite robust tests that are already present for the urlparser
module. Unfortunately there's no good way to be 100% certain that
this works for all the many ways that Paste can do an import. So
this patch is mostly hoping that test coverage is sufficient.

[1] https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly

Fixes #75

Author: cdent

paste/urlparser.py

@@ -7,7 +7,7 @@
 import os
 import six
 import sys
-import imp
+import importlib.util as imputil
 import mimetypes
 try:
     import pkg_resources
@@ -376,7 +376,6 @@ def load_module_from_name(environ, filename, module_name, errors):
             return None
         f.write('#\n')
         f.close()
-    fp = None
     if module_name in sys.modules:
         return sys.modules[module_name]
     if '.' in module_name:
@@ -386,14 +385,16 @@ def load_module_from_name(environ, filename, module_name, errors):
                                        parent_name, errors)
     else:
         base_name = module_name
-    fp = None
-    try:
-        fp, pathname, stuff = imp.find_module(
-            base_name, [os.path.dirname(filename)])
-        module = imp.load_module(module_name, fp, pathname, stuff)
-    finally:
-        if fp is not None:
-            fp.close()
+    module = None
+
+    # This replaces a previous implementation that used imp with the canonical
+    # steps from importlib.
+    spec = imputil.spec_from_file_location(base_name, filename)
+    if spec is not None:
+        module = imputil.module_from_spec(spec)
+        sys.modules[base_name] = module
+        spec.loader.exec_module(module)
+
     return module
 
 def make_py(parser, environ, filename):