make install-for-dev has version conflict on filelock

[nhoening]

pip-compile -o requirements/dev.txt requirements/dev.in will result in a dependency problem:

There are incompatible versions in the resolved dependencies:
  filelock==3.8.0 (from -c requirements/app.txt (line 82))
  filelock<4,>=3.11 (from virtualenv==20.22.0->pre-commit==2.11.0->-r requirements/dev.in (line 4))

I could resolve it with adding this to my plugin's requrements/dev.in:

pre-commit < 2.11.1
virtualenv==20.0.8

This is a quick-fix. We can accept it and move on. A real solution could:

• sort out which of these pins are the best
• ponder if adding the fixed requirements of a specific flexmeasures version in plugins often leads to problems with other dependencies, and if installing flexmeasures should maybe be done via the .in dependencies, rather than .txt ... This would be a general FlexMeasures issue, of course.