Expose `.data._symcache` stuff at subpkg toplevel

The list is `open_symcache()`, `get_symcache()`, `SymbologyCache`, and
`Stuct` which seems more or less fine to make part of the public
namespace. Also, make `._timeseries.t_unit` an instance of literal to make
`ruff` happy?

piker/data/__init__.py

@@ -39,9 +39,15 @@ from .feed import (
     open_feed,
 )
 from .flows import Flume
+from ._symcache import (
+    SymbologyCache,
+    open_symcache,
+    get_symcache,
+)
+from .types import Struct
 
 
-__all__ = [
+__all__: list[str] = [
     'Flume',
     'Feed',
     'open_feed',
@@ -53,4 +59,8 @@ __all__ = [
     'get_shm_token',
     'def_iohlcv_fields',
     'def_ohlcv_fields',
+    'open_symcache',
+    'get_symcache',
+    'SymbologyCache',
+    'Struct',
 ]

piker/data/_timeseries.py

@@ -225,7 +225,7 @@ def detect_null_time_gap(
     return None
 
 
-t_unit: Literal[
+t_unit: Literal = Literal[
     'days',
     'hours',
     'minutes',

piker/data/flows.py

@@ -31,7 +31,6 @@ import pendulum
 import numpy as np
 
 from ..accounting import MktPair
-from ._util import log
 from .types import Struct
 from ._sharedmem import (
     attach_shm_array,
@@ -233,5 +232,3 @@ class Flume(Struct):
             np.all(np.isin(vlm, -1))
             or np.all(np.isnan(vlm))
         )
-
-