minor: Fix minor warning on unused import

[viirya]

Which issue does this PR close?

Closes #.

Rationale for this change

Got a warning when verifying 20.0.0 RC 2 release:

warning: unused import: `py_runtime_err` 386/387: datafusion-python                                                                                                                                                                  
  --> src/expr/literal.rs:18:21
   |
18 | use crate::errors::{py_runtime_err, DataFusionError};
   |                     ^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default


warning: 1 warning emitted===========> ] 386/387: datafusion-python                                                                                                                                                                  

What changes are included in this PR?

Are there any user-facing changes?

[andygrove]

Thanks @viirya

[viirya]

Thanks @andygrove