RFC for ergonomically passing Python variables to Julia

I propose one of the following two APIs as a way to more ergonomically pass Python variables to Julia code:

```python
jl.teval("const MY_CONST = $(x)", x=my_python_const)
```

a "template" eval, or

```python
with jl.let(x=my_python_const):
jl.seval("const MY_CONST = x")
```
which is a juliacall "let."

Would be interested in hearing other ideas too.

This is designed to address the following issue. Currently, passing Python objects to Julia via `seval` requires creating closure functions:

```python
jl.seval("x -> @eval const MY_CONST = $x")(my_python_const)
```

While Python's PEP 750 proposes [Template Strings](https://peps.python.org/pep-0750/) which could offer one solution:

```python
jl.seval(t"const MY_CONST = {my_python_const}")
```

We could implement something similar with a new `teval` method:

```python
jl.teval("const MY_CONST = {my_python_const}")
```

However, this has two key issues:

1. The `{}` syntax conflicts with Julia type signatures (e.g., `Vector{Float64}`), which could cause subtle bugs. Even if t-strings are eventually merged to Python, we would still face this issue!
2. `teval` wouldn't have access to local variables to reference `my_python_const`

Let's look at the two proposed approaches in more detail:

The first approach makes variable passing explicit but ergonomic:

```python
jl.teval("const MY_CONST = $(x)", x=my_python_const)
```

The second approach uses a context manager for temporary variable binding:

```python
with jl.let(x=my_python_const):
jl.seval("const MY_CONST = x") # Would access the bound value
```

Basically the `seval` would have access to this stack of Python objects passed via `let`, and put them into the Julia context via a Julia-evaluated `let` statement.

Thoughts? @cjdoris @mkitti

Going to post this on the Julia discourse too. Edit: https://discourse.julialang.org/t/rfc-for-ergonomically-passing-python-variables-to-julia/124266

Start by reviewing the two proposed APIs, teval and let, along with the linked Julia Discourse discussion. Done would require an agreed approach for ergonomically passing Python variables into Julia and a concrete implementation scope.

Written by the indexing model from the issue text.