Add 3.6 and 3.7 to classifiers

[Misha Balyasin]

We test in 3.7-3.9, so makes sense to add 3.7 to classifiers
Should also add 3.6 to matrix to check if tests still pass there and if yes - add it to classifiers as well.

[Misha Balyasin]

pandas doesn't support 3.7, so no longer relevant