fix: Access jax.config from jax (#2376)

* As of jax and jaxlib v0.4.20 accessing jax.config from the jax.config
  submodule is deprecated and it should be accessed from the jax top
  level API instead.

   ```
   >>> from jax.config import config
   <stdin>:1: DeprecationWarning: Accessing jax.config via the jax.config submodule is deprecated
   ```

src/pyhf/tensor/jax_backend.py

@@ -1,4 +1,4 @@
-from jax.config import config
+from jax import config
 
 config.update('jax_enable_x64', True)