Xlcalculator

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- Use a less intrusive way to patch ``openpyxl``. Instead of permanently
patching the reader to support cached formula values, ``mock`` is used to
only patch the reader while reading the workbook.

This way the patches do not interfere with other packages not expecting
these new classes.

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- Support for delayed node evaluation by wrapping them into expressions. The
function will eval the expression when needed.

- Support for native Excel data types.

- Enable and update Excel file based function tests that are now working
properly.

- Flake8 source code.

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- Refactored ``xlcalculator`` types to be more compact.

- Reimplemented evaluation engine to not generate Python code anymore, but
build a proper AST from the AST nodes. Each AST node supports an `eval()`
function that knows how to compute a result.

This removes a lot of complexities around trying to determine the evaluation
context at code creation time and encoding the context as part of the
generated code.

- Removal of all special function handling.

- Use of new `xlfunctions` implementation.

- Use Openpyxl to load the Excel files. This provides shared formula support
for free.

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- Initial release.