Avoid converting to `int` in `calculate_block_difficulty`

[SamWilsn]

In pre-merge forks, the calculate_block_difficulty function converts to int before doing the calculations.

Ideally, we should try and use types from ethereum-types whenever possible to indicate the intended signedness and bitwidth.

[omegajudith]

Hi @SamWilsn ,

I’d like to contribute to this issue if it’s still available. I went through a couple of other issues but saw they were either already assigned or had someone asking to work on them, so I thought I’d check take this up.

Please let me know if I can go ahead and start working on it. Excited to contribute!

[omegajudith]

Just a quick update here @SamWilsn : I'm starting to work on this issue now.
I'll open a PR referencing this once I have something ready.

[SamWilsn]

Thank you! To save yourself a lot of headache, I'd recommend implementing only a single fork (eg. homestead) and then opening up a draft pull request to get feedback.

[omegajudith]

Alright @SamWilsn! . Iam right on it

[omegajudith]

@SamWilsn please asign

[omegajudith]

Hi @SamWilsn: I’ve opened a Draft PR for #1415, scoped to Homestead only, refactoring calculate_block_difficulty to remove redundant int() casts while preserving behavior (same bomb math and clamp to MINIMUM_DIFFICULTY).

Local checks
• tox -e static ✅
• tox -e py3 -- -k 'homestead and difficulty' ✅
• Full filler on my box: tox -e py3 ✅

Could you take a quick look to confirm the approach before I mirror it to later forks?
PR link: #1460