reports: Fix mypy errors.

Errors were:

  conservancy_beancount/reports/core.py:923: error: Generator has incompatible
  item type "Optional[str]"; expected "str"

  conservancy_beancount/reports/core.py:929: error: Item "None" of
  "Optional[str]" has no attribute "lower"

  conservancy_beancount/reports/ledger.py:534: error: Item "None" of
  "Optional[str]" has no attribute "partition"

  conservancy_beancount/reports/ledger.py:729: error: Item "None" of
  "Optional[str]" has no attribute "lower"

  conservancy_beancount/reports/rewrite.py:563: error: Argument 2 to
  "_iter_yaml" of "RewriteRuleset" has incompatible type "Union[Any, str,
  None]"; expected "str"

conservancy_beancount/reports/core.py

@@ -920,13 +920,13 @@ class BaseODS(BaseSpreadsheet[RT, ST], metaclass=abc.ABCMeta):
         if not flags:
             raise ValueError(f"no valid edges in {edges!r}")
         border_attr = f'{width} {style} {color}'
-        key = f'{",".join(f.name for f in flags)} {border_attr}'
+        key = f'{",".join(str(f.name) for f in flags)} {border_attr}'
         try:
             retval = self._style_cache[key]
         except KeyError:
             props = odf.style.TableCellProperties()
             for flag in flags:
-                props.setAttribute(f'border{flag.name.lower()}', border_attr)
+                props.setAttribute(f'border{str(flag.name).lower()}', border_attr)
             retval = odf.style.Style(
                 name=f'Border{next(self._name_counter)}',
                 family='table-cell',

conservancy_beancount/reports/ledger.py

@@ -531,7 +531,7 @@ class ReportType(enum.IntFlag):
             return cls.DEBIT_TRANSACTIONS
 
     def _choices_sortkey(self) -> Sortable:
-        subtype, _, maintype = self.name.partition('_')
+        subtype, _, maintype = str(self.name).partition('_')
         return (maintype, subtype)
 
 
@@ -726,7 +726,7 @@ date was also not specified.
         type=report_type.enum_type,
         default=report_type_default,
         help=f"""The type of report to generate. Choices are
-{report_type.choices_str()}. Default is {report_type_default.name.lower()!r}.
+{report_type.choices_str()}. Default is {str(report_type_default.name).lower()!r}.
 """)
     # --transactions got merged into --report-type; this is backwards compatibility.
     parser.add_argument(

conservancy_beancount/reports/rewrite.py

@@ -553,7 +553,7 @@ class RewriteRuleset:
             if isinstance(source, str):
                 name = '<string>'
             else:
-                name = getattr(source, 'name', '<file>')
+                name = str(getattr(source, 'name', '<file>'))
         try:
             doc = yaml.safe_load(source)
         except yaml.error.YAMLError as error: