Drop Support for Python 3.6

[samineer]

Reasons

1. django-ftl doesn't support Python 3.6.
2. Since 2021-12-23 Python 3.6 is EOL (see PEP 494).
3. Type annotation improvements in Python 3.7 (see PEP 563).

Possibly changes

• Use dataclasses instead of attrs.
• Use builtin dict instead of collections.OrderedDict.

See also What’s New In Python 3.7.

[spookylukey]

If possible, I'd rather not drop Python 3.6 yet - for one thing, because fluent.runtime still supports it, and I'd like fluent-compiler to be a drop in replacement. For the level of type annotations required for #14 (i.e. user facing type annotations, rather than internals), I don't think we'll need much that is only available in Python 3.7. Is there something that can't be typed easily in Python 3.6 that we need, and is available in 3.7?

[samineer]

Is there something that can't be typed easily in Python 3.6 that we need, and is available in 3.7?

No, there's nothing. In Python 3.7 added the optional feature that type annotations are not executed at runtime, which improves performance.

I agree with you! So I'm closing this issue.