use choices to restrict -A/--api

also documents what APIs are supported in `--help`

gwdatafind/__main__.py

@@ -25,9 +25,9 @@ import igwn_segments as segments
 
 from . import (
     __version__,
+    api,
     ui,
 )
-from .api import DEFAULT_API
 from .io import (
     format_cache,
     lal_cache,
@@ -232,7 +232,8 @@ def command_line():
     sargs.add_argument(
         "-A",
         "--api",
-        default=DEFAULT_API,
+        default=api.DEFAULT_API,
+        choices=api.APIS,
         help="API version to use",
     )