Doc: Allow building documentation even without a compiled extension

This involves a fake _ldap module and carefully bootstrapping
ldap.__init__ (which imports * from _ldap).

Doc/conf.py

@@ -12,9 +12,16 @@
 # serve to show the default value.
 
 import sys
+import os
 
 # If your extensions are in another directory, add it here.
-#sys.path.append('some/directory')
+_doc_dir = os.path.dirname(__file__)
+sys.path.append(_doc_dir)
+sys.path.append(os.path.join(_doc_dir, '../Lib/'))
+sys.path.insert(0, os.path.join(_doc_dir, '../Lib/ldap'))
+
+# Import fake `_ldap` module
+import fake_ldap_module_for_documentation
 
 # General configuration
 # ---------------------

Doc/fake_ldap_module_for_documentation.py

@@ -0,0 +1,30 @@
+"""
+A module that mocks `_ldap` for the purposes of generating documentation
+
+This module provides placeholders for the contents of `_ldap`, making it
+possible to generate documentation even _ldap is not compiled.
+It should also make the documentation independent of which features are
+available in the system OpenLDAP library.
+
+The overly long module name will show up in AttributeError messages,
+hinting that this is not the actual _ldap.
+
+See https://www.python-ldap.org/ for details.
+"""
+
+import sys
+
+# Cause `import _ldap` to import this module instead of the actual `_ldap`.
+sys.modules['_ldap'] = sys.modules[__name__]
+
+from constants import CONSTANTS
+from pkginfo import __version__
+
+for constant in CONSTANTS:
+    globals()[constant.name] = constant
+
+def get_option(num):
+    pass
+
+class LDAPError:
+    pass

Lib/ldap/constants.py

@@ -10,6 +10,9 @@
 
 """
 
+# This module cannot import anything from ldap.
+# When building documentation, it is used to initialize ldap.__init__.
+
 from __future__ import print_function
 
 class Constant(object):