Add failing test for conda/pip conflict

[maresb]

Added @jamesmyatt's environment file as a failing test from #540 (comment).

I'd really like to know the veracity of the following assertion at the end of conda_lock.lockfile.v2prelim.models.Lockfile._toposort:

assert len(final_package) == len(package)

Running this test in the debugger, the assertion holds, and dep.manager != manager is never triggered. (It's also currently not triggered by anything in the test suite. 😢)

I expect that figuring out the right thing to do here and fixing it will involve quite a deep dive.

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