Expose export_spec from hdmf.spec.write for import from pynwb.spec (#1092)

* Expose export_spec from hdmf.spec.write for import from pynwb.spec

* Fix import

Author: rly

src/pynwb/spec.py

@@ -2,6 +2,7 @@
 
 from hdmf.spec import LinkSpec, GroupSpec, DatasetSpec, SpecNamespace,\
                        NamespaceBuilder, AttributeSpec, DtypeSpec, RefSpec
+from hdmf.spec.write import export_spec  # noqa: F401
 from hdmf.utils import docval, get_docval, fmt_docval_args
 
 from . import CORE_NAMESPACE