Pin importlib-metadata dependency to maintain CLI compatibility for python 3.7

[dfangl]

Problem

importlib-metadata recently had their 5.0 major version update, where they removed some "compatibility shims" (https://importlib-metadata.readthedocs.io/en/latest/history.html). I think it is, more concretely, python/importlib_metadata#284 which lead to the incompatibility.
This lead to stevedore returning an error, since it tries to access the entrypoints like a dict, and therefore in python 3.7 (3.8 and later use importlib.metadata directly) the CLI to start LS did not work anymore.
Since stevedore abandoned python3.7 with its latest version 4.0.0, the chances of a fix are slim (even though the dict-like access might be removed at some point), so for now a dependency pin has to suffice.

Solution

Pin the dependency of importlib-metadata to <5.0 for now. Since this dependency is only used in python3.7 or lower, the pin is only applicable for those versions.

[coveralls]

Coverage decreased (-0.008%) to 79.069% when pulling e79f5dd on fix-python37-cli into 4b8318e on master.

[github-actions]

LocalStack integration with Pro

       3 files  ±0         3 suites  ±0   1h 13m 37s ⏱️ - 1m 49s
1 339 tests ±0  1 217 ✔️ ±0  122 💤 ±0  0 ❌ ±0 
1 853 runs  ±0  1 585 ✔️ ±0  268 💤 ±0  0 ❌ ±0 

Results for commit e79f5dd. ± Comparison against base commit 4b8318e.

[thrau]

excellent! love the version constraint