fix(build): use stricter requirement syntax

https://setuptools.pypa.io/en/latest/userguide/declarative_config.html#opt-2

In install_requires and extras_require, values are parsed as list-semi. This implies that in order to include markers, each requirement must be dangling in a new line:

```
[options]
install_requires =
    importlib-metadata; python_version<"3.8"

[options.extras_require]
all =
    importlib-metadata; python_version < "3.8"
```

setup.cfg

@@ -51,8 +51,10 @@ console_scripts =
     scriv = scriv.cli:cli
 
 [options.extras_require]
-toml = tomli;python_version<3.11
-yaml = pyyaml
+toml =
+    tomli; python_version < "3.11"
+yaml =
+    pyyaml
 
 [scriv]
 ghrel_template = file: ghrel_template.md.j2