feat(cli): write "date-time" with dash

"datetime" will look strange to a user who doesn't know Python. It
exposes internals that may change. (What if we adopt a special class for
times without dates?) Let us instead pick something very close from
natural language.

README.md

@@ -84,8 +84,8 @@ options:
                         input format
   --json-indent n, --indent-json n
                         JSON indentation
-  -k, --stringify       Turn into strings boolean, datetime, and null keys for
-                        JSON and TOML and null values for TOML
+  -k, --stringify       Turn into strings boolean, date-time, and null keys
+                        for JSON and TOML and null values for TOML
   -o output, --output output
                         output file
   --of {cbor,json,msgpack,toml,yaml}, -of {cbor,json,msgpack,toml,yaml}, --output-format {cbor,json,msgpack,toml,yaml}

remarshal.py

@@ -139,7 +139,7 @@ def parse_command_line(argv: List[str]) -> argparse.Namespace:  # noqa: C901.
             dest="stringify",
             action="store_true",
             help=(
-                "Turn into strings boolean, datetime, and null keys for JSON "
+                "Turn into strings boolean, date-time, and null keys for JSON "
                 "and TOML and null values for TOML"
             ),
         )
@@ -438,7 +438,7 @@ def reject_special_keys(key: Any) -> Any:
         msg = "boolean key"
         raise TypeError(msg)
     if isinstance(key, datetime.datetime):
-        msg = "datetime key"
+        msg = "date-time key"
         raise TypeError(msg)
     if key is None:
         msg = "null key"