force recreation of venv if it is not sane (#286)

[finswimmer]

With this PR a virtual environment is recreated if it's not sane, meaning the python and pip executable is not found within the venv path.

I'm not sure how to write a test for it. If it's necessary, please give me a hint :)

Fixes: #286

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[sdispater]

We might go into a loop here since there is no guarantee that recreating the virtual environment will fix the issue. If we go this route we should at least display a warning that we recreate the environment because it's invalid.

Another approach, or an addition to this PR, could be to check if the environment is valid after creating it and, if it's not, we could warn the user. A lot of cases we had here comes from the fact that Debian-based systems do not include the venv module by default which leads to a broken environment. If it's only pip missing, we could try to install it by using the get-pip.py script.

[finswimmer]

I've added a more explicit warning.

I also have a variant in preparation where I check the sanity of the venv after creating this. But I have a problem to pass the tests in env/test_use, because the build_venv is mocked and just create a folder without any content. I'm not sure yet, how to fix these tests...

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