Use external toml package to support Python 3.10

pyproject.toml

@@ -24,6 +24,7 @@ dependencies = [
 	"pandas",
 	"mkdocstrings[python]",
   "xarray",
+  "toml"
 ]
 
 classifiers = [

src/echosms/referencemodels.py

@@ -1,7 +1,7 @@
 """Reference model parameters."""
 
 from pathlib import Path
-import tomllib
+import toml  # Could use standard library tomllib once python >=3.11
 import pandas as pd
 pd.options.mode.copy_on_write = True
 
@@ -25,8 +25,8 @@ def __init__(self):
 
         self.definitions = []
 
-        with open(self.defs_filename, 'rb') as f:
-            self.definitions = tomllib.load(f)
+        with open(self.defs_filename, 'r') as f:
+            self.definitions = toml.load(f)
 
         # Flag duplicate target names
         pda = pd.Series(self.names())