[lldb] Configure pyright to the documented minimum python version (llvm#162952)

Pyright is an MIT-licensed static type checker and can be found at
    https://github.com/microsoft/pyright
there are also various integrations to use it as an LSP server in
various editors which is the main way I use it.

It's useful on our python scripts to detect issues such as where
functions are called with unexpected types or it's possible to access
obj.attr on an object that doesn't have that attribute. It can be used
without any configuration this config setting causes it to also report
issues with type hints that do not meet our python 3.8 minimum such as
this one from dap_server.py:
```
        init_commands: list[str],
```
subscripting the builtin type like that requires python 3.9 while the
3.8 equivalent is:
```
from typing import List
...
        init_commands: List[str],
```
In practice these scripts still work on 3.8 because type hints aren't
normally evaluated during normal execution but since we have a minimum,
we should fully comply with it.

Note: The error pyright reports for this particular issue isn't great:
```
error: Subscript for class "list" will generate runtime exception; enclose type expression in quotes
```
This is technically correct as it is possible to evaluate type hints at
runtime but I believe anything that would do so would also evaluate the
string form as well and still hit the runtime exception. A better
suggestion in this case would have been the 3.8 compatible `List[str]`.
However, it is better than silently passing code that doesn't confirm to
the minimum.

Author: dsandersllvm

pyproject.toml

@@ -4,3 +4,8 @@ extend-exclude = '''
   third-party/
 )
 '''
+
+[tool.pyright]
+executionEnvironments = [
+  { root = "lldb/packages/Python", pythonVersion = "3.8" }
+]