Switch AnthropicTokenizer to use client.beta.messages.count_tokens()

[yifanmai]

When anthropic is upgraded to 0.39.0 or newer, the following error happens:

src/helm/tokenizers/anthropic_tokenizer.py:26: error: "Anthropic" has no attribute "get_tokenizer"  [attr-defined]

This is because of this breaking change:

### ⚠ BREAKING CHANGES

* **client:** remove legacy `client.count_tokens()` & `client.get_tokenizer()` methods ([#726](https://github.com/anthropics/anthropic-sdk-python/issues/726))
  * This functionality has been replaced by the `client.beta.messages.count_tokens()` API which supports newer models and all content functionality, such as images and PDFs.

The temporary workaround is to limit anthropic to versions before 0.39.0. The long term change is to limit anthropic to after 0.39.0 and switch AnthropicTokenizer to use client.beta.messages.count_tokens().