Generate requirements.txt from Python spec (apache#7289)

* Generate requirements.txt from Python spec.

* add tests, collect actual requirements (first cut).

* add tornado and cloudpickle

* add xgboost

* add xgboost version restriction

* cleanup and prepare for merge

* black format

* add type annotations and docstrings

* remove example requirements.txt

* fix setup.py extras_require

* use typing. classes for type annotations, python 2 compatible :)

* fix python2 typing.Pattern

* retrigger CI

* address comaniac comments

* retrigger ci

python/.gitignore

@@ -1,3 +1,4 @@
 build
 dist
 *.cpp
+requirements/*.txt