Test against Flynt master branch in the CI build

[akaihola]

Fixes #473

[akaihola]

In the CI build, strangely I see all three packages collected:

Collecting black@ git+https://github.com/psf/black.git@main (from -r constraints-future.txt (line 6))
Collecting flynt@ git+https://github.com/ikamensh/flynt.git@master (from -r constraints-future.txt (line 7))
Collecting isort@ git+https://github.com/PyCQA/isort.git@main (from -r constraints-future.txt (line 8))

but only Black built:

Building wheels for collected packages: black
  Building wheel for black (pyproject.toml): started
  Building wheel for black (pyproject.toml): finished with status 'done'
  Created wheel for black: filename=black-24.3.1.dev2+gf78b157-py3-none-any.whl size=202064 sha256=9b361d2f13af64d86a0fa5da5b164710c92f3446567edc24ede5f14c332983c9
  Stored in directory: /tmp/pip-ephem-wheel-cache-n1bid6p8/wheels/8a/42/78/08c93525cd9d7cbe48f98f80f893f1fbb4a572510b8657e725
Successfully built black
Installing collected packages: black
  Attempting uninstall: black
    Found existing installation: black 24.3.0
    Uninstalling black-24.3.0:
      Successfully uninstalled black-24.3.0
Successfully installed black-24.3.1.dev2+gf78b157

I added a pip list after upgrading those packages, let's see if they still are build and installed even though the log doesn't show it.

[akaihola]

Indeed, isort and flynt are not upgraded to main/master:

Package                 Version
----------------------- --------------------
[...]
black                   24.3.1.dev2+gf78b157
[...]
flynt                   1.0.1
[...]
isort                   5.13.2
[...]

[akaihola]

Ok so Black uses hatch to create development version numbers, while isort and flynt don't. So the version of isort and flynt in master/main is the same as for the last release. That's why pip install -U wouldn't upgrade them.

So I added a pip uninstall -y -r ... before pip install -r .... Seems to work, uninstall just ignores the version specifiers.