Loading repo from explicit hash fails due to fix for other issues in custom nomenclature version

[korsbakken]

Specifying git_hash parameter in iam_validation.nomenclature.defs.NomenclatureDefs.from_url fails with a git error. The reason appears to be that the loading code in the custom version of the nomenclature-iamc package currently specified in pyproject.toml tries to pull updates, but that fails if the id is an explicit hash rather than a branch name. We need to either fix the fix, or complete the PR to the nomenclature-iamc that ensures non-ASCII characters in region mappings don't cause crashes PR #406.

[korsbakken]

The relevant PR in nomenclature has been merged there, and is included in release 0.20.0. Commit 43ebc3c (release v0.1.0) here therefore switches the dependency to the released nomenclature-iamc package, which closes this issue.