Add comprehensive type annotations and Python 3 modernization

MANIFEST.in

@@ -1,3 +1,5 @@
 include README.rst
 include LICENSE
 include lensfunpy/version_helper.h
+include lensfunpy/py.typed
+include lensfunpy/_lensfun.pyi

dev-requirements.txt

@@ -11,6 +11,7 @@ scipy
 
 # test dependencies
 pytest
+mypy  # for type checking and stubtest validation
 
 # documentation dependencies
 sphinx_rtd_theme

lensfunpy/__init__.py

@@ -1,4 +1,14 @@
-from __future__ import absolute_import
+from __future__ import absolute_import, annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from lensfunpy._lensfun import (
+        Database, Camera, Mount, Lens, Modifier,
+        ModifyFlags, LensType, DistortionModel, TCAModel, VignettingModel,
+        LensCalibDistortion, LensCalibTCA, LensCalibVignetting,
+        LensfunError, XMLFormatError
+    )
 
 from ._version import __version__