Use constant for valid delimiters

Avoids re-defining this list at each use case and prevents them from
getting out of sync.

augur/io/metadata.py

@@ -12,6 +12,10 @@
 from .file import open_file
 
 
+# List of valid delimiters when reading a metadata file.
+VALID_DELIMITERS = (',', '\t')
+
+
 def read_metadata(metadata_file, id_columns=("strain", "name"), chunk_size=None):
     """Read metadata from a given filename and into a pandas `DataFrame` or
     `TextFileReader` object.
@@ -135,7 +139,6 @@ def read_table_to_dict(table, duplicate_reporting=DataErrorMethod.ERROR_FIRST, i
         2. The provided *id_column* does not exist in the *metadata*
         3. The *duplicate_reporting* method is set to ERROR_FIRST or ERROR_ALL and duplicate(s) are found
     """
-    valid_delimiters = [',', '\t']
     seen_ids = set()
     duplicate_ids = set()
     with open_file(table) as handle:
@@ -151,7 +154,7 @@ def read_table_to_dict(table, duplicate_reporting=DataErrorMethod.ERROR_FIRST, i
         try:
             # Note: this sort of duplicates _get_delimiter(), but it's easier if
             # this is separate since it handles non-seekable buffers.
-            dialect = csv.Sniffer().sniff(table_sample, valid_delimiters)
+            dialect = csv.Sniffer().sniff(table_sample, VALID_DELIMITERS)
         except csv.Error as err:
             raise AugurError(
                 f"Could not determine the delimiter of {table!r}. "
@@ -435,7 +438,7 @@ def _get_delimiter(path: str):
     with open_file(path) as file:
         try:
             # Infer the delimiter from the first line.
-            return csv.Sniffer().sniff(file.readline(), delimiters=[',', '\t']).delimiter
+            return csv.Sniffer().sniff(file.readline(), VALID_DELIMITERS).delimiter
         except csv.Error as err:
             raise AugurError(
                 f"Could not determine the delimiter of {path!r}. "