Cl-sii

- (PR 45, 2019-05-15) libs.encoding_utils: improve `clean_base64`
- (PR 44, 2019-05-15) dte.parse: fix edge case in `parse_dte_xml`

- (PR 40, 2019-05-08) dte.data_models: fix bug in `DteDataL2`

Includes backwards-incompatible changes to data model `DteDataL2`.

- (PR 38, 2019-05-08) dte.data_models: alter field `DteDataL2.signature_x509_cert_pem`
- (PR 37, 2019-05-08) dte.data_models: alter field `DteDataL2.firma_documento_dt_naive`
- (PR 36, 2019-05-08) libs.crypto_utils: add functions
- (PR 35, 2019-05-07) libs.tz_utils: minor improvements
- (PR 34, 2019-05-06) docs: Fix `bumpversion` command

- (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"