(core) Convert row tuples to lists to fix excel import error

Summary:
openpyxl was producing tuples while some older code expects lists. Choosing to convert the tuples to lists (instead of making the other code work with tuples) in case there's other similar issues still out there. Should fix the error mentioned in https://grist.slack.com/archives/C0234CPPXPA/p1652797247167719:

```
Traceback (most recent call last):
  File "/gristroot/grist/sandbox/grist/sandbox.py", line 103, in run
    ret = self._functions[fname](*args)
  File "/gristroot/grist/sandbox/grist/imports/register.py", line 11, in parse_excel
    return import_file(file_source)
  File "/gristroot/grist/sandbox/grist/imports/import_xls.py", line 20, in import_file
    parse_options, tables = parse_file(path)
  File "/gristroot/grist/sandbox/grist/imports/import_xls.py", line 26, in parse_file
    return parse_open_file(f)
  File "/gristroot/grist/sandbox/grist/imports/import_xls.py", line 69, in parse_open_file
    table_data_with_types = parse_data.get_table_data(rows, len(headers))
  File "/gristroot/grist/sandbox/grist/parse_data.py", line 215, in get_table_data
    row.extend([""] * missing_values)
AttributeError: 'tuple' object has no attribute 'extend'
```

Test Plan: Existing tests. Haven't figured out how to reproduce the original error.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3434

sandbox/grist/imports/import_xls.py

@@ -41,7 +41,7 @@ def parse_open_file(file_obj):
   for sheet in workbook:
     table_name = sheet.title
     rows = [
-      row
+      list(row)
       for row in sheet.iter_rows(values_only=True)
       # Exclude empty rows, i.e. rows with only empty values.
       # `if not any(row)` would be slightly faster, but would count `0` as empty.