internal: do not hardcode MSRV in README badge

[chirizxc]

btw, instead of hardcoding MSRV, we can get it from https://crates.io/api/v1/crates/pyo3 (versions[0].rust_version)

[davidhewitt]

The dynamic versioning is quite convenient. I don't quite understand the need to switch from shields.io, is there a particular motivation other than a design refresh? Can we do that with some theming on shields.io?

[chirizxc]

Can we do that with some theming on shields.io?

[![minimum rustc](https://img.shields.io/badge/dynamic/json?url=https://crates.io/api/v1/crates/pyo3&query=$.versions[0].rust_version&label=rustc&suffix=%2B&color=blue&logo=rust)](https://rust-lang.github.io/rfcs/2495-min-rust-version.html)

[davidhewitt]

👍 in which case I think:

• let's stick to shields.io, I see no reason to move
• the dynamic rust version looks handy given we often forget to update it and all the other badges are dynamic
• I'm ok with new themes, if shields.io supports it and you want to propose some

[chirizxc]

👍 in which case I think:

* let's stick to shields.io, I see no reason to move

* the dynamic rust version looks handy given we often forget to update it and all the other badges are dynamic

* I'm ok with new themes, if shields.io supports it and you want to propose some

done