Return type for _ExpandIntoRanges DoFn should be Iterable. (#22548)

The following fails otherwise:

```
import apache_beam as beam
from apache_beam.io import textio
with beam.Pipeline(options=beam.options.pipeline_options.PipelineOptions(
        runtime_type_check=True, type_check_additional='all')) as p:
    p | beam.Create(['/bin/sh']) | textio.ReadAllFromText()
```

```
apache_beam.typehints.typehints.CompositeTypeHintError: Union[FileMetadata, OffsetRange] type-constraint violated. Expected an instance of one of: ('FileMetadata', 'OffsetRange'), received tuple instead.
```

Author: jonathanasdf

sdks/python/apache_beam/io/filebasedsource.py

@@ -29,6 +29,7 @@
 # pytype: skip-file
 
 from typing import Callable
+from typing import Iterable
 from typing import Tuple
 from typing import Union
 
@@ -347,7 +348,7 @@ def __init__(
     self._compression_type = compression_type
 
   def process(self, element: Union[str, FileMetadata], *args,
-              **kwargs) -> Tuple[FileMetadata, OffsetRange]:
+              **kwargs) -> Iterable[Tuple[FileMetadata, OffsetRange]]:
     if isinstance(element, FileMetadata):
       metadata_list = [element]
     else: