MNT: Apply Repo-Review suggestions

Apply some Repo-Review suggestions.

I went for the low-hanging fruits, other changes are more invasive and deserve a PR of their own, such as using a formatter like ruff of black.

Also update minimal Python version for blue. By the way, isn't blue able to understand requires-python?

MY103: MyPy warn unreachable Must have warn_unreachable = true to pass this check. There are occasionally false positives (often due to platform or Python version static checks), so it's okay to ignore this check. But try it first - it can catch real bugs too.
```
[tool.mypy]
warn_unreachable = true
```
MY104: MyPy enables ignore-without-code Must have "ignore-without-code" in enable_error_code = [...]. This will force all skips in your project to include the error code, which makes them more readable, and avoids skipping something unintended.
```
[tool.mypy]
enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
```
MY105: MyPy enables redundant-expr Must have "redundant-expr" in enable_error_code = [...]. This helps catch useless lines of code, like checking the same condition twice.
```
[tool.mypy]
enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
```
MY106: MyPy enables truthy-bool Must have "truthy-bool" in enable_error_code = []. This catches mistakes in using a value as truthy if it cannot be falsey.
```
[tool.mypy]
enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
```

Left for later because it requires extensive fixes:

MY101: MyPy strict mode Must have strict in the mypy config. MyPy is best with strict or nearly strict configuration. If you are happy with the strictness of your settings already, ignore this check or set strict = false explicitly.
```
[tool.mypy]
strict = true
```

nipy / nibabel