RuntimeError when using flair 0.8 with Python 3.9

[b2m]

Describe the bug

RuntimeError when trying to use flair==0.8 with Python 3.9.

To Reproduce

On Windows using Powershell:

$ python -m venv venv
$ .\venv\Scripts\activate
$ python --version
> Python 3.9.2
$ pip install --no-cache-dir flair==0.8
> ...
$ python
>>> from flair.data import Sentence
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "E:\code\flair\venv\lib\site-packages\flair\__init__.py", line 2, in <module>
    import torch
  File "E:\code\flair\venv\lib\site-packages\torch\__init__.py", line 196, in <module>
    from torch._C import *
RuntimeError: module compiled against API version 0xe but this version of numpy is 0xd

On Docker using bash in the python:3.9 image:

$ python --version
> 3.9.2
$ pip install flair==0.8
> ...
$ python
>>> from flair.data import Sentence
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/site-packages/flair/__init__.py", line 20, in <module>
    from . import models
  File "/usr/local/lib/python3.9/site-packages/flair/models/__init__.py", line 1, in <module>
    from .sequence_tagger_model import SequenceTagger, MultiTagger
  File "/usr/local/lib/python3.9/site-packages/flair/models/sequence_tagger_model.py", line 21, in <module>
    from flair.embeddings import TokenEmbeddings, StackedEmbeddings, Embeddings
  File "/usr/local/lib/python3.9/site-packages/flair/embeddings/__init__.py", line 6, in <module>
    from .token import TokenEmbeddings
  File "/usr/local/lib/python3.9/site-packages/flair/embeddings/token.py", line 13, in <module>
    import gensim
  File "/usr/local/lib/python3.9/site-packages/gensim/__init__.py", line 5, in <module>
    from gensim import parsing, corpora, matutils, interfaces, models, similarities, summarization, utils  # noqa:F401
  File "/usr/local/lib/python3.9/site-packages/gensim/corpora/__init__.py", line 6, in <module>
    from .indexedcorpus import IndexedCorpus  # noqa:F401 must appear before the other classes
  File "/usr/local/lib/python3.9/site-packages/gensim/corpora/indexedcorpus.py", line 15, in <module>
    from gensim import interfaces, utils
  File "/usr/local/lib/python3.9/site-packages/gensim/interfaces.py", line 21, in <module>
    from gensim import utils, matutils
  File "/usr/local/lib/python3.9/site-packages/gensim/matutils.py", line 1104, in <module>
    from gensim._matutils import logsumexp, mean_absolute_difference, dirichlet_expectation
  File ".eggs/Cython-0.29.14-py2.7-linux-x86_64.egg/Cython/Includes/numpy/__init__.pxd", line 242, in init gensim._matutils
ValueError: numpy.ndarray size changed, may indicate binary incompatibility. Expected 88 from C header, got 80 from PyObject

Environment (please complete the following information):

• OS: Windows 10 and Docker with python:3.9 image.
• Version: flair-0.8

Additional context

• I fixed this problem locally by installing numpy==1.20.0 (instead of the required 1.19.0).
• Not reproducible with the Docker images python:3.8, python:3.7 and python:3.6.