Do not require ast_serialize to avoid transitive dependency on Rust

[barracuda156]

ast_serialize requires Rust, which does not support every platform. Requiring it unnecessarily makes it impossible to install mypy in such cases.

Upd. If actually was added as an optional dependency in 40ffeda but then sneakily turned into required one.

[barracuda156]

@ilevkivskyi Could you please take a look? It will be really upsetting to have mypy broken now. Earlier versions (through 1.20.2) installed and worked fine.

P. S. Before someone says that it is not needed to have Rust installed on a given machine: yes, that’s true, but it does not help if Rust itself is broken on a given platform to begin with.