Fix deprecation warning with newer Pythons and 'imp' library.

This warning is shown:
```
/opt/appuser/.local/lib/python3.9/site-packages/pyhocon/config_parser.py:4: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
    import imp

-- Docs: https://docs.pytest.org/en/stable/warnings.html
```

`imp.find_module()` (https://docs.python.org/3/library/imp.html#imp.find_module) was
deprecated since version 3.3.
However Pyhocon has no support for Python 3.3. We can use the alternative function
`importlib.util.find_spec()` (https://docs.python.org/3/library/importlib.html#importlib.util.find_spec)
which was added in Python 3.4 as advised in the docs.

See:#248

pyhocon/config_parser.py

@@ -1,7 +1,6 @@
 import codecs
 import contextlib
 import copy
-import imp
 import itertools
 import logging
 import os
@@ -63,6 +62,27 @@ def glob(pathname, recursive=False):
 else:
     from glob import glob
 
+
+# Fix deprecated warning with 'imp' library and Python 3.4+.
+# See: https://github.com/chimpler/pyhocon/issues/248
+if sys.version_info >= (3, 4):
+    import importlib.util
+
+    def find_package_dir(name):
+        spec = importlib.util.find_spec(name)
+        # When `imp.find_module()` cannot find a package it raises ImportError.
+        # Here we should simulate it to keep the compatibility with older
+        # versions.
+        if not spec:
+            raise ImportError('No module named {!r}'.format(name))
+        return os.path.dirname(spec.origin)
+else:
+    import imp
+
+    def find_package_dir(name):
+        return imp.find_module(name)[1]
+
+
 logger = logging.getLogger(__name__)
 
 #
@@ -727,7 +747,7 @@ def resolve_package_path(cls, package_path):
         if ':' not in package_path:
             raise ValueError("Expected format is 'PACKAGE:PATH'")
         package_name, path_relative = package_path.split(':', 1)
-        package_dir = imp.find_module(package_name)[1]
+        package_dir = find_package_dir(package_name)
         path_abs = os.path.join(package_dir, path_relative)
         return path_abs