Agate-excel

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* Add an ``encoding_override`` argument to :meth:`.Table.from_xls` to override the encoding of the input XLS file.
* Add a ``header`` argument to :meth:`.Table.from_xls` and :meth:`.Table.from_xlsx` to indicate the presence of a header row.
* Add a ``read_only`` argument to :meth:`.Table.from_xlsx` to allow disabling read-only mode for `some spreadsheets <https://openpyxl.readthedocs.io/en/stable/optimized.html#worksheet-dimensions>`_.

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* Overload :meth:`.Table.from_xls` and :meth:`.Table.from_xlsx` to accept and return multiple sheets.
* Add a ``skip_lines`` argument to :meth:`.Table.from_xls` and :meth:`.Table.from_xlsx` to skip rows from the top of the sheet.
* Fix bug in handling ambiguous dates in XLS. (9)
* Fix bug in handling an empty XLS.
* Fix bug in handling non-string column names in XLSX.

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* Fix bug in handling of ``None`` in boolean columns for XLS. (11)
* Removed usage of deprecated openpyxl method ``get_sheet_by_name``.
* Remove monkeypatching.
* Upgrade required agate version to ``1.5.0``.
* Ensure columns with numbers for names (e.g. years) are parsed as strings.

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* Initial version.