plugins.handlers: use stdlib for EntryPointPlugin.get_version()

`importlib.metadata` officially starts existing in Python 3.8, and we
are removing support for Python 3.7 before Sopel 8's stable release.
That means we can use the stdlib approach here to get the version string
for an entry-point plugin, and drop another `import importlib_metadata`.

I can't wait for Python 3.9's EOL... We can remove `importlib_metadata`
completely then.

sopel/plugins/handlers.py

@@ -52,9 +52,6 @@
 import sys
 from typing import Optional, TYPE_CHECKING, TypedDict
 
-# TODO: refactor along with usage in sopel.__init__ in py3.8+ world
-import importlib_metadata
-
 from sopel import __version__ as release, loader, plugin as plugin_decorators
 from . import exceptions
 
@@ -614,9 +611,13 @@ def get_version(self) -> Optional[str]:
         """
         version: Optional[str] = super().get_version()
 
-        if version is None and hasattr(self.module, "__package__"):
+        if (
+            version is None
+            and hasattr(self.module, "__package__")
+            and self.module.__package__ is not None
+        ):
             try:
-                version = importlib_metadata.version(self.module.__package__)
+                version = importlib.metadata.version(self.module.__package__)
             except ValueError:
                 # package name is probably empty-string; just give up
                 pass