Support for python env managers

[lfoppiano]

This PR adds support for python virtual environments for JDK 11+. Should be merged after #921.
It supports venv and conda.

Update: I've noticed I'm not able to run grobid natively on linux without installing the python dependencies as a system dependencies. Usually I've correlate the use of this PR with the removal of the JEP library under the grobid-home, in favour of the JEP installed in the python library / virtual environment.

We could improve the detection of the python version.

[coveralls]

coverage: 40.768% (-0.01%) from 40.781%
when pulling 8a16401 on python-envs
into 37dcec1 on master.

[lfoppiano]

With this PR is possible to run a different version than 3.8 (the jep shipped with grobid requires python 3.8) and relay on conda for the environment.

Here what I did for running DL models natively using a conda environment:

1. cd grobid
2. Switch to this branch
3. rm -rf grobid-home/lib/lin-64/jep
4. cd ..
5. git clone https://github.com/kermitt2/delft (delft should be in the parent directory)
6. assuming conda is installed (if not, I installed this) and the environment delft has been created with either python=3.8 or python=3.9 (e.g. conda create --name delft python=3.8
7. pip install delft==0.3.4
8. conda install jep=4.1.1 (conda will install also openjdk 8)
9. conda uninstall --force openjdk-8
10. conda install --force openjdk-17
11. change any model in the grobid config to use delft instead of wapiti (e.g. header model)
12. ./gradlew run