Work around no unused ignores

Seems pretty silly, but since it can't be turned off on a line-by-line
basis this is probably better than changing the mypy config.

Author: mvdbeek

lib/galaxy/util/compression_utils.py

@@ -17,6 +17,7 @@
     Optional,
     overload,
     Tuple,
+    TYPE_CHECKING,
     Union,
 )
 
@@ -31,10 +32,13 @@
     is_gzip,
 )
 
-try:
+if TYPE_CHECKING:
     from isal import isal_zlib
-except ImportError:
-    isal_zlib = None  # type: ignore[assignment]
+else:
+    try:
+        from isal import isal_zlib
+    except ImportError:
+        isal_zlib = None
 
 
 log = logging.getLogger(__name__)