Fix mypy errors in test_backends.py (pydata#10689)

- Add explicit keyword arguments to netCDF4.Dataset.createVariable calls
- Add type: ignore comments for dynamic compression parameter
- Remove unnecessary type: ignore for requests import

These changes resolve mypy type checking errors that were causing CI failures.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: max-sixty

.gitignore

@@ -89,3 +89,4 @@ doc/videos-gallery.txt
 # gitignore to make it _easier_ to work with `uv`, not as an indication that I
 # think we shouldn't...)
 uv.lock
+mypy_report/

xarray/tests/test_backends.py

@@ -206,11 +206,17 @@ def _check_compression_codec_available(codec: str | None) -> bool:
 
             # Attempt to create a variable with the compression
             if codec and codec.startswith("blosc"):
-                nc.createVariable(
-                    "test", "f4", ("x",), compression=codec, blosc_shuffle=1
+                nc.createVariable(  # type: ignore[call-overload]
+                    varname="test",
+                    datatype="f4",
+                    dimensions=("x",),
+                    compression=codec,
+                    blosc_shuffle=1,
                 )
             else:
-                nc.createVariable("test", "f4", ("x",), compression=codec)
+                nc.createVariable(  # type: ignore[call-overload]
+                    varname="test", datatype="f4", dimensions=("x",), compression=codec
+                )
 
             nc.close()
             os.unlink(tmp_path)