Support Rust 1.47.0 for Ubuntu

[gaborbernat]

Describe the bug
Fails to compile on docker image ubuntu:latest.

To Reproduce

cargo install python-launcher

   Compiling python-launcher v0.15.0
error[E0658]: use of unstable library feature 'bool_to_option'
   --> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/python-launcher-0.15.0/src/cli.rs:164:29
    |
164 |         venv_path.is_file().then(|| venv_path)
    |                             ^^^^
    |
    = note: see issue #64260 <https://github.com/rust-lang/rust/issues/64260> for more information
error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.
error: could not compile `python-launcher`.

Expected behaviour
The build to succeed.

System Details (please complete the following information):

• OS: ubuntu:latest
• Shell: bash
• Launcher Version: latest

rust (1.47) & cargo (0.47) installed via apt-get. Note it works if one installs the latest rust via https://rustup.rs.

[brettcannon]

I have only worried about compatibility with stable (https://github.com/brettcannon/python-launcher#how-do-i-install-the-launcher), so I'm considering this a feature request to backport to Rust 1.47.0.

The specific API in question is bool::then() which went stable in Rust 1.50.

[gaborbernat]

Would be nice if cargo would fail here that rust 1.50 or later is required 🤔