Ocrd-calamari

- Fix: Depend on protobuf < 4.0 (mikegerber, 89)
- Tested on Python 3.6-3.10

- README: Do not mention obsolete + unsupported checkpoint parameter (mikegerber, 80)
- Skip tiny or bin-empty lines (bertsky, 76)

- 🚨 **Breaking change**: There is no `checkpoint` parameter anymore, it was replaced with the new parameter `checkpoint_dir` (bertsky, 70)
- e.g. `-P checkpoint "/var/lib/calamari-models/GT4HistOCR/2019-12-11T11_10+0100/*.ckpt.json"` becomes `-P checkpoint_dir "/var/lib/calamari-models/GT4HistOCR/2019-12-11T11_10+0100/"`
- Descend to all available TextRegions recursively (bertsky, 75)
- This release is now tested on Python 3.6 → 3.10 (mikegerber)

* Fix TensorFlow dependency for Python 3.9
* init from constructor not process() (bertsky)

* Warn if passing raw images to single-channel models (bertsky)
* Fix word and glyph coordinates (mikegerber)

* ocrd_calamari now supports OCR-D resource resolving, i.e. search paths for models (kba)
* New parameter `checkpoint_dir`, to give model directory (not model files) (kba)
* Bug fix: handle empty line images 48 (kba)