Csvw

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- Better CSVW spec conformance:
- `csvw.CSVW.to_json` implements https://w3c.github.io/csvw/csv2json/
- `csvw.CSVW.is_valid`
- Completed datatype support
- CLI
- Docs

Backwards incompatibility:
- `commentPrefix=None` for default CSV dialect for Table and TableGroup (instead of ``)
- duplicate column names in CSVW metadata now raise `ValueError` rather than just issuing a warning
- Dropped Python 3.6 compatibility

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- Fix CSVW spec compatibility: Stop doing blackslash escaping/doubling with
(default) Dialect(doubleQuote=True) (https://github.com/cldf/csvw/issues/7).
- Fix doubling of backslashes with csv.QUOTE_NONE
(https://github.com/cldf/csvw/issues/4).
- Python 3.10 support.

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- Dropped py35 compat.
- Enhanced foreign key validation.

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- fixed bug where the line number was not correctly reported by
UnicodeReaderWithLineNumber when multi-line cell content was read.
- extended tests to python 3.9

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- Allow reading of data and metadata from URLs.

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- Read all data with encoding specified as UTF-8 using utf-8-sig encoding.