Cl-sii

- (PR 32, 2019-05-03) Requirements: updates and package upper-bounds

- (PR 29, 2019-04-25) dte.data_models: modify new fields of `DteDataL2`
- (PR 28, 2019-04-25) libs: add module `crypto_utils`
- (PR 27, 2019-04-25) libs: add module `encoding_utils`
- (PR 26, 2019-04-25) test_data: add files
- (PR 25, 2019-04-25) libs.xml_utils: fix class alias `XmlElementTree`
- (PR 24, 2019-04-25) requirements: add and update packages
- (PR 22, 2019-04-24) test_data: add files
- (PR 21, 2019-04-22) dte: many improvements
- (PR 20, 2019-04-22) libs.xml_utils: misc improvements
- (PR 19, 2019-04-22) test_data: fix and add real SII DTE & AEC XML files
- (PR 18, 2019-04-22) data.ref: add XML schemas for "Cesion" (RTC)

- (PR 16, 2019-04-16) dte.parse: change and improve `clean_dte_xml`
- (PR 14, 2019-04-09) data.ref: merge XML schemas dirs
- (PR 13, 2019-04-09) extras: add Marshmallow field for a DTE's "tipo DTE"

- (PR 11, 2019-04-05) dte: add module `parse`
- (PR 10, 2019-04-05) dte: add module `data_models`
- (PR 9, 2019-04-05) libs: add module `xml_utils`
- (PR 8, 2019-04-05) add sub-package `rcv`

- (PR 6, 2019-04-04) data.ref: add XML schemas of "factura electrónica"
- (PR 5, 2019-04-04) extras: add 'RutField' for Django models, DRF and MM
- (PR 4, 2019-04-04) Config CircleCI

- (PR 2, 2019-04-04) Add class and constants for RUT
- (PR 1, 2019-04-04) Whole setup for a Python package/library