Added Python 3.10 Linux jobs

[radarhere]

After matthew-brett/trusty#13 and matthew-brett/trusty#17, and updating multibuild to include https://github.com/matthew-brett/multibuild/pull/423 to change MB_ML_VER to 2014 as suggested in https://github.com/matthew-brett/multibuild/issues/421#issuecomment-898354903, this PR adds Python 3.10 jobs for Linux.

[hugovk]

Thank you!

For PyPy3/Linux: do we need to update MB_ML_VER to 2014 and DOCKER_TEST_IMAGE to focal in build.sh?

if [[ "$MB_PYTHON_VERSION" == pypy3* ]]; then
  if [[ "$TRAVIS_OS_NAME" != "macos-latest" ]]; then
    MB_ML_VER="2010"
    DOCKER_TEST_IMAGE="multibuild/xenial_$PLAT"
  else
    MB_PYTHON_OSX_VER="10.9"
  fi
fi

And this is likely a longer term multibuild question and not immediately relevant here: should it use something newer than Trusty 14.04?

[radarhere]

https://github.com/pypa/manylinux

manylinux2010 (CentOS 6 based - EOL)

Since 2010 is EOL, updating to MB_ML_VER to 2014 seems like a good idea, sure.

[radarhere]

As has been said, there aren't focal docker images for 32-bit. Is dropping support for 32-bit Linux wheels something that we're interested in doing? At the very least, I didn't think that this PR was the right place for that.

Regarding other options, there are some problems at the moment -

• For 64-bit, if I try updating DOCKER_TEST_IMAGE to focal for Linux PyPy, ...something isn't right. PLAT isn't being passed in for some reason - https://github.com/radarhere/pillow-wheels/runs/3430741826

/usr/bin/choose_python.sh: line 68: python: command not found

(edit: added in #218)

• If I try xenial instead, there's another problem - https://github.com/radarhere/pillow-wheels/runs/3432713147?check_suite_focus=true#step:4:6972

Features should be: 'fribidi harfbuzz raqm transp_webp webp_anim webp_mux xcb'; but are 'transp_webp webp_anim webp_mux xcb'

• xenial is also not working for Python 3.10 currently - https://github.com/radarhere/pillow-wheels/runs/3432713284#step:4:6600 (edit: it seems there isn't interest in supporting Python 3.10 in the images, Update pip to 21.2.3 for Python 3.10 multi-build/docker-images#12 (comment))

AttributeError: module 'sysconfig' has no attribute 'get_preferred_scheme'

So this PR is the simplest option for the moment.

[hugovk]

The point of this PR is to add 3.10 for Linux, so agreed best to keep it simple and let's merge this. Thanks for trying that lot out.

Dropping 32-bit: something to consider, download numbers are pretty low, but let's discuss it another time in the main repo.

Thanks again!