Brightway2

* Update to ecoinvent 3.7 flows

* Fix openpyxl extraction giving formulas instead of numerical values

* Fix [83](https://github.com/brightway-lca/brightway2-io/issues/83): Skip data that can't be exported to Excel.
* Fix [85](https://github.com/brightway-lca/brightway2-io/issues/85): Inconsistent `categories` types in base data

* Switch from xlrd to [openpyxl](https://openpyxl.readthedocs.io/en/stable/) for reading `.xlsx` files.
* Added Excel and CSV importer for LCIA methods
* Merged [PR 77](https://github.com/brightway-lca/brightway2-io/pull/77): Add synonyms when importing ecospold2 files. Thanks BenPortner.
* Merged [PR 76](https://github.com/brightway-lca/brightway2-io/pull/76): Correctly import reference products from Excel. Thanks BenPortner.
* Import exchange properties from ecospold2 files.
* `bw2io.extractors.excel.ExcelExtractor` now properly handles internal Excel errors.

* Switch to openpyxl for xlsx imports
* Port Ben Portner's fixes for CSV encoding and newline handling
* Add extraction of ecospold2 exchange properties
* PR [72](https://github.com/brightway-lca/brightway2-io/pull/72): expose `objs` argument in `write_lci_csv`
* Handle Excel error values correctly when extracting

Fix bug in importing sample database (missing files)