Don't use TypeVar when defining coordinate types. (#150)

My previous change which used TypeVar for coordinate types [1] was
actually a Bad Idea.  While it was better in that it prohibited
heterogeneous collections and slices, it was worse in that made the
SparseDFCoord type (and derived types) into generic types, which
we do not want.

The *right* way to avoid repeating all the possible types is to make
a union of the possible *containers*, and then specify *that* with all
the possible contents that may go into those containers.

[1]: #141

python-spec/src/somacore/options.py

@@ -110,20 +110,7 @@ class ResultOrder(enum.Enum):
 ResultOrderStr = Union[ResultOrder, Literal["auto", "row-major", "column-major"]]
 """A ResultOrder, or the str representing it."""
 
-# Coordinate types are specified as TypeVars instead of Unions to ensure that
-# sequences and slices are homogeneous:
-#
-#     BadCoord = Union[int, str]
-#     BadCoords = Union[BadCoord, Sequence[BadCoord]]
-#     # ["this", 1] is bad, but is a valid as a BadCoords value
-#
-#     GoodCoord = TypeVar("GoodCoord", int, str)
-#     GoodCoords = Union[GoodCoord, Sequence[GoodCoord]]
-#     # ["this", 1] is bad, and is *not* valid as a GoodCoords value
-#     # ["this", "that"] is a valid as a GoodCoords value
-#     # [1, 2] is valid as a GoodCoords value
-
-DenseCoord = TypeVar("DenseCoord", None, int, types.Slice[int])
+DenseCoord = Union[None, int, types.Slice[int]]
 """A single coordinate range for reading dense data.
 
 ``None`` indicates the entire domain of a dimension; values of this type are
@@ -134,18 +121,21 @@ class ResultOrder(enum.Enum):
 DenseNDCoords = Sequence[DenseCoord]
 """A sequence of ranges to read dense data."""
 
-SparseCoord = TypeVar(
-    "SparseCoord", bytes, float, int, slice, str, np.datetime64, pa.TimestampType
-)
-"""Types that can be put into a single sparse coordinate."""
+_T = TypeVar("_T")
+ValSliceOrSequence = Union[_T, types.Slice[_T], types.Sequence[_T]]
+"""A value of a type, a Slice of that type, or a Sequence of that type."""
 
 # NOTE: Keep this in sync with the types accepted in `_canonicalize_coord`
 # in ./query/axis.py.
 SparseDFCoord = Union[
-    DenseCoord,
-    SparseCoord,
-    Sequence[SparseCoord],
-    types.Slice[SparseCoord],
+    None,
+    ValSliceOrSequence[bytes],
+    ValSliceOrSequence[float],
+    ValSliceOrSequence[int],
+    ValSliceOrSequence[slice],
+    ValSliceOrSequence[str],
+    ValSliceOrSequence[np.datetime64],
+    ValSliceOrSequence[pa.TimestampType],
     pa.Array,
     pa.ChunkedArray,
     npt.NDArray[np.integer],
@@ -156,13 +146,11 @@ class ResultOrder(enum.Enum):
 """A sequence of coordinate ranges for reading dense dataframes."""
 
 SparseNDCoord = Union[
-    DenseCoord,
-    Sequence[int],
+    None,
+    ValSliceOrSequence[int],
     npt.NDArray[np.integer],
     pa.IntegerArray,
 ]
 """A single coordinate range for one dimension of a sparse nd-array."""
-
-
 SparseNDCoords = Sequence[SparseNDCoord]
 """A sequence of coordinate ranges for reading sparse ndarrays."""