Disable coverage for impossible cases

repo_management/operations.py:
Do not try to cover the cases, that coveragepy is not able to cover
(e.g. async for loops with awaitables).
Upstream coveragepy seems to still have issues coverying different kinds
of concurrency techniques:
nedbat/coveragepy#1012

repo_management/operations.py

@@ -31,7 +31,7 @@ async def db_file_as_models(
     packages: Dict[str, models.OutputPackageBase] = {}
     package_descs: Dict[str, models.PackageDesc] = {}
     package_files: Dict[str, models.Files] = {}
-    async for member in files._db_file_member_as_model(
+    async for member in files._db_file_member_as_model(  # pragma: no cover
         db_file=await files._read_db_file(db_path=db_path, compression=compression)
     ):
         if member.member_type == defaults.RepoDbMemberType.DESC:
@@ -99,7 +99,7 @@ async def create_db_from_json_files(
 
     repodbfile = convert.RepoDbFile()
     database = await files._write_db_file(path=output_path)
-    async for path in files._json_files_in_directory(path=input_path):
+    async for path in files._json_files_in_directory(path=input_path):  # pragma: no cover
         model = await files._read_pkgbase_json_file(path)
         await files._stream_package_base_to_db(
             db=database,