feat: Allow smart_open 7.x (#2745)

### Description

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Increases the maximum version bound on `smart_open` to `<8.0` to allow
the current `7.x` releases.

https://github.com/piskvorky/smart_open/releases/tag/v7.0.1

The release notes don’t seem to call out any significant breaking
changes.

### QC
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* [x] The PR contains a test case for the changes or the changes are
already covered by an existing test case. **Regression-testing should be
covered by the existing tests.**
* [x] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake). **I don’t
think any documentation changes are needed.**

It’s difficult for me to run the full set of tests offline, so I’m
relying on CI to double-check this.

setup.cfg

@@ -50,7 +50,7 @@ install_requires =
     pyyaml
     requests >=2.8.1,<3.0
     reretry
-    smart-open >=3.0,<7.0
+    smart-open >=3.0,<8.0
     snakemake-interface-executor-plugins >=9.0.0,<10.0
     snakemake-interface-common >=1.17.0,<2.0
     snakemake-interface-storage-plugins >=3.1.0,<4.0