[Core] Add deprecation warnings about removal of shared libraries. (#3106)

* feat: add deprecation warning when importing shared libs

* enhance(downloader): add shared libs deprecation warns

* enhance: add deprecation warning when (re)loading cogs

* docs(downloader): add deprecation note about shared libs

* chore(changelog): add towncrier entries

* style: split long tuple unpacks in multiple lines

* fix: argument to `humanize_list` has to be a sequence

redbot/core/_sharedlibdeprecation.py

@@ -0,0 +1,29 @@
+from importlib.abc import MetaPathFinder
+import warnings
+
+
+class SharedLibDeprecationWarning(DeprecationWarning):
+    pass
+
+
+warnings.simplefilter("always", SharedLibDeprecationWarning)
+
+
+class SharedLibImportWarner(MetaPathFinder):
+    """
+    Deprecation warner for shared libraries. This class sits on `sys.meta_path`
+    and prints warning if imported module is a shared library
+    """
+
+    def find_spec(self, fullname, path, target=None) -> None:
+        """This is only supposed to print warnings, it won't ever return module spec."""
+        parts = fullname.split(".")
+        if parts[0] != "cog_shared" or len(parts) != 2:
+            return None
+        msg = (
+            "One of cogs uses shared libraries which are"
+            " deprecated and scheduled for removal in Red 3.3.\n"
+            "You should inform author of the cog about this message."
+        )
+        warnings.warn(msg, SharedLibDeprecationWarning, stacklevel=2)
+        return None