Fix mypy error for csv.Sniffer.sniff(delimiter)

The type of the delimiter parameter to csv.Sniffer.sniff is str, so
that's what should be accepted here.

Also check to ensure that delimiters are single characters.

augur/io/metadata.py

@@ -456,10 +456,15 @@ def write_records_to_tsv(records, output_file):
 
 def _get_delimiter(path: str, valid_delimiters: Iterable[str]):
     """Get the delimiter of a file given a list of valid delimiters."""
+
+    for delimiter in valid_delimiters:
+        if len(delimiter) != 1:
+            raise AugurError(f"Delimiters must be single-character strings. {delimiter!r} does not satisfy that condition.")
+
     with open_file(path) as file:
         try:
             # Infer the delimiter from the first line.
-            return csv.Sniffer().sniff(file.readline(), valid_delimiters).delimiter
+            return csv.Sniffer().sniff(file.readline(), "".join(valid_delimiters)).delimiter
         except csv.Error as error:
             # This assumes all csv.Errors imply a delimiter issue. That might
             # change in a future Python version.