Update species.py for Rat GRCr8

merges #26

Author: ACastanza

ensembl_genes/species.py

@@ -78,8 +78,8 @@ def get_mhc_category(self, gene: GeneForMHC) -> str | None:
 rat = Species(
     name="rattus_norvegicus",
     common_name="rat",
-    # mRatBN7.2
-    assembly="72",
+    # GRCr8
+    assembly="1",
     # https://github.com/related-sciences/ensembl-genes/issues/4#issuecomment-941556912
     ensembl_gene_pattern=r"^ENSRNOG[0-9]{11}$",
     # FIXME: mhc coordinates