[pygobject] Fix ctypes.PyDLL construction under Windows

[Christoph Reiter <creiter src gnome org>]

commit af9ddf322f7cf377996b8fa29906bd4b254dc001
Author: Christoph Reiter <creiter src gnome org>
Date:   Fri Dec 8 16:40:33 2017 +0100

    Fix ctypes.PyDLL construction under Windows
    
    We require the ctypes.pythonapi interface but can't use the global one
    since we have to change it which could potentially break other users
    of that interface.
    
    Turns out simply passing None to PyDLL() only works on Unix to load the
    CPython library. Instead copy the logic from the ctypes module in the
    stdlib.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=622084

 gi/_ossighelper.py |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)
---
diff --git a/gi/_ossighelper.py b/gi/_ossighelper.py
index 0fde1bd..2c72134 100644
--- a/gi/_ossighelper.py
+++ b/gi/_ossighelper.py
@@ -139,7 +139,18 @@ def wakeup_on_signal():
             _wakeup_fd_is_active = False
 
 
-pydll = ctypes.PyDLL(None)
+def create_pythonapi():
+    # We need our own instance of ctypes.pythonapi so we don't modify the
+    # global shared one. Adapted from the ctypes source.
+    if os.name == "nt":
+        return ctypes.PyDLL("python dll", None, sys.dllhandle)
+    elif sys.platform == "cygwin":
+        return ctypes.PyDLL("libpython%d.%d.dll" % sys.version_info[:2])
+    else:
+        return ctypes.PyDLL(None)
+
+
+pydll = create_pythonapi()
 PyOS_getsig = pydll.PyOS_getsig
 PyOS_getsig.restype = ctypes.c_void_p
 PyOS_getsig.argtypes = [ctypes.c_int]