Make python-ldap optional for PyPI packages

python-ldap is a Python package with heavy C extensions. In order to
build python-ldap, not only OpenLDAP development headers are necessary,
but also OpenSSL, Cyrus SASL, and MIT KRB5 development headers.

A fully functional ipaclient doesn't need an LDAP driver. It talks JSON
RPC over HTTPS to a server. python-ldap is only used by ipapython.dn.DN
to convert a string to a DN with ldap_str2dn(). The function is simple
and can be wrapped with ctypes in a bunch of lines.

Related: https://pagure.io/freeipa/issue/6468
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipapython/dn.py

@@ -423,10 +423,16 @@ import sys
 import functools
 
 import cryptography.x509
-from ldap.dn import str2dn, dn2str
-from ldap import DECODING_ERROR
 import six
 
+try:
+    from ldap import DECODING_ERROR
+except ImportError:
+    from ipapython.dn_ctypes import str2dn, dn2str, DECODING_ERROR
+else:
+    from ldap.dn import str2dn, dn2str
+
+
 if six.PY3:
     unicode = str