- Use tag, not random sha

- update to 4.19.1:
* Mark package as compatible with Sphinx 3.1.
* Refactoring. #528
* Make debug config variables available in conf.py. #533
* Fix warning formatting for function lookup. #535
* Correctly reverse nested namespaces in get_qualification. #540

- Increase required version of Sphinx to the one where module
cstructobject was introduced. (bsc#1168627)

- version update to 4.18.1
* 2020-05-10 - Breathe v4.18.1
- Fix friend class rendering and allow friend struct. #522
- Add extern examples to doc and remove variable hack. #526
- Render function candidates without using Sphinx directives. #524
* 2020-05-02 - Breathe v4.18.0
- Support tiles in verbatim blocks. #517
* 2020-05-01 - Breathe v4.17.0
- Scoped rendering, better integration with Sphinx, misc fixes. #512
* 2020-04-19 - Breathe v4.16.0
- Strictly depend on Sphinx's minor version. #498
- Simplifications and fixes, use more of modern Sphinx natively. #503
- Add section option to the doxygen(auto)file directive. #501
- Fix link generation when enum is inside a group (enum FQDN). #508
- Fix creation of LaTeX math formulas. #506
- Improve documentation for doxygen(auto)file section option. #509
* 2020-04-07 - Breathe v4.15.0
- Add license file to distribution. #492