chore: unpin fgpyo dependency

Does `fgpyo` need to be pinned to `0.7.1`, or can we use our usual [caret dependency](https://python-poetry.org/docs/dependency-specification/#caret-requirements) to permit any `0.x` minor version at 0.8 or later? 

I'd at least like to bump the version to 0.8.0 - it'd be helpful in a client project to have `fgpyo.io.assert_fasta_indexed()` available, which was added in 0.8.0.

pyproject.toml

@@ -37,7 +37,7 @@ python = "^3.11"
 pyproject_hooks= "^1.0.0,!=1.1.0"
 pybedlite = "1.0.0"
 strenum = "^0.4.15"
-fgpyo = "0.7.1"
+fgpyo = "^0.8.0"
 pysam = "^0.22.1"
 ordered-set = "^4.1.0"