Loosen very strict version pinning

Hey flatlanders,

you use very strict version pinning, e.g. pytest==3.8.2. This makes it hard to satisfy the requirements when you use flatland in a bigger project which has their own requirements. I guess in most cases the exact version does not really matter for flatland. Maybe you can loosen the version requirements a bit.

Also, some versions are old-ish, e.g. pytest is 5.0.0 since 2019-06-28 but you require 3.8.2.