export_v2: Make "none" an invalid coloring key

Update `augur export v2` to warn about the invalid coloring "none" key
and skip it in export so that it does not include a coloring key that
will clash with the recent change in Auspice to use "none" to hide
tip labels¹.

¹ nextstrain/auspice#1618

augur/export_v2.py

@@ -377,6 +377,9 @@ def _is_valid(coloring):
         if key != "gt" and not trait_values:
             warn("You asked for a color-by for trait '{}', but it has no values on the tree. It has been ignored.".format(key))
             return False
+        if key == "none":
+            warn(f"You asked for a color-by for trait {key!r}, but this is an invalid coloring key. It has been ignored.")
+            return False
         return True
 
     def _get_colorings():

tests/functional/export_v2.t

@@ -165,5 +165,23 @@ Run export with metadata and external colors TSV that contains zero values.
   {}
   $ rm -f "$TMP/dataset6.json"
 
+Run export with metadata that contains "none" column and asked to use as coloring.
+This is expected to output a warning that "none" is an invalid coloring column and skip it in colorings.
+
+  $ ${AUGUR} export v2 \
+  >  --tree export_v2/tree.nwk \
+  >  --metadata export_v2/dataset1_metadata_with_none_column.tsv \
+  >  --node-data export_v2/div_node-data.json export_v2/location_node-data.json \
+  >  --auspice-config export_v2/auspice_config5.json \
+  >  --maintainers "Nextstrain Team" \
+  >  --output "$TMP/dataset7.json" > /dev/null
+  WARNING: You asked for a color-by for trait 'none', but this is an invalid coloring key. It has been ignored.
+  \s{0} (re)
+
+  $ python3 "$TESTDIR/../../scripts/diff_jsons.py" export_v2/dataset1.json "$TMP/dataset7.json" \
+  >   --exclude-paths "root['meta']['updated']" "root['meta']['maintainers']"
+  {}
+  $ rm -f "$TMP/dataset1.json"
+
 
   $ popd > /dev/null

tests/functional/export_v2/auspice_config5.json

@@ -0,0 +1,37 @@
+{
+  "colorings": [
+    {
+      "key": "location",
+      "title": "Location",
+      "type": "categorical",
+      "legend": [
+        {"value": "alpha", "display": "α"},
+        {"value": "beta"}
+      ],
+      "scale": [
+        ["beta", "#bd0026"],
+        ["gamma", "#6a51a3"]
+      ]
+    },
+    {
+      "key": "mutation_length",
+      "title": "Mutations per branch",
+      "type": "continuous",
+      "legend": [
+        {"value": 1, "display": "0-2", "bounds": [-1,2]},
+        {"value": 3, "display": "3-5", "bounds": [2,5]},
+        {"value": 5, "display": ">5", "bounds": [5, 10]}
+      ],
+      "scale": [
+        [1, "#081d58"],
+        [3, "#1d91c0"],
+        [5, "#c7e9b4"]
+      ]
+    },
+    {
+      "key": "none",
+      "title": "None",
+      "type": "boolean"
+    }
+  ]
+}

tests/functional/export_v2/dataset1_metadata_with_none_column.tsv

@@ -0,0 +1,7 @@
+strain	div	mutation_length	none
+tipA	1	1	A
+tipB	3	1	B
+tipC	3	1	C
+tipD	8	3	D
+tipE	9	4	E
+tipF	6	1	F