Specify an MSRV in setup.py

[alex]

No description provided.

[alex]

Here's the output with this specified:

$ pip install -e .
[...]
    error: Rust 1.41.1 does not match extension requirement >=1.45.0
    
        =============================DEBUG ASSISTANCE=============================
        If you are seeing a compilation error please try the following steps to
        successfully install cryptography:
        1) Upgrade to the latest pip and try again. This will fix errors for most
           users. See: https://pip.pypa.io/en/stable/installing/#upgrading-pip
        2) Read https://cryptography.io/en/latest/installation.html for specific
           instructions for your platform.
        3) Check our frequently asked questions for more information:
           https://cryptography.io/en/latest/faq.html
        4) Ensure you have a recent Rust toolchain installed:
           https://cryptography.io/en/latest/installation.html#rust
        5) If you are experiencing issues with Rust for *this release only* you may
           set the environment variable `CRYPTOGRAPHY_DONT_BUILD_RUST=1`.
        =============================DEBUG ASSISTANCE=============================

[reaperhulk]

Do we want to ship this in a 3.4.3?

[alex]

I think so. I'll prep a backport PR.