Update legacy Python image for CI tests in Linux
There is an exception when running `pip install .[tests]`:
```
Processing /home/circleci/datasets
Collecting numpy>=1.17 (from datasets==1.12.2.dev0)
  Downloading https://files.pythonhosted.org/packages/45/b2/6c7545bb7a38754d63048c7696804a0d947328125d81bf12beaa692c3ae3/numpy-1.19.5-cp36-cp36m-manylinux1_x86_64.whl (13.4MB)
    100% |████████████████████████████████| 13.4MB 3.9MB/s  eta 0:00:011

...

Collecting faiss-cpu (from datasets==1.12.2.dev0)
  Downloading https://files.pythonhosted.org/packages/87/91/bf8ea0d42733cbb04f98d3bf27808e4919ceb5ec71102e21119398a97237/faiss-cpu-1.7.1.post2.tar.gz (41kB)
    100% |████████████████████████████████| 51kB 30.9MB/s ta 0:00:01
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "/home/circleci/.pyenv/versions/3.6.14/lib/python3.6/site-packages/setuptools/sandbox.py", line 154, in save_modules
        yield saved
      File "/home/circleci/.pyenv/versions/3.6.14/lib/python3.6/site-packages/setuptools/sandbox.py", line 195, in setup_context
        yield
      File "/home/circleci/.pyenv/versions/3.6.14/lib/python3.6/site-packages/setuptools/sandbox.py", line 250, in run_setup
        _execfile(setup_script, ns)
      File "/home/circleci/.pyenv/versions/3.6.14/lib/python3.6/site-packages/setuptools/sandbox.py", line 45, in _execfile
        exec(code, globals, locals)
      File "/tmp/easy_install-1pop4blm/numpy-1.21.2/setup.py", line 34, in <module>
        method can be invoked.
    RuntimeError: Python version >= 3.7 required.
```

Apparently, `numpy-1.21.2` tries to be installed in the temporary directory `/tmp/easy_install-1pop4blm` instead of the downloaded `numpy-1.19.5` (requirement of `datasets`).

This is caused because `pip` downloads the `.tar.gz` (instead of the `.whl`) and tries to build it in a tmp dir.
Instead of legacy, use next-generation convenience images, built from the ground up with CI, efficiency, and determinism in mind. Here are some of the highlights:

- Faster spin-up time - In Docker terminology, these next-gen images will generally have fewer and smaller layers. Using these new images will lead to faster image downloads when a build starts, and a higher likelihood that the image is already cached on the host.

- Improved reliability and stability - The existing legacy convenience images are rebuilt practically every day with potential changes from upstream that we cannot always test fast enough. This leads to frequent breaking changes, which is not the best environment for stable, deterministic builds. Next-gen images will only be rebuilt for security and critical-bugs, leading to more stable and deterministic images.

More info: https://circleci.com/docs/2.0/circleci-images