Replace deprecated pkg_resources with stdlib (#239)

`pkg_resources` has been deprecated by `setuptools` for quite a while: https://setuptools.pypa.io/en/latest/pkg_resources.html

It's got some bugs/warts:

* pypa/setuptools#2531
* https://discuss.python.org/t/will-setuptools-remove-pkg-resource-module-in-the-future/27182

So switch to using `importlib` functions which are part of the Python standard library as of `3.8`.

This is less error-prone, and also removes the need for `setuptools` to be installed in order for this panel to work.

I realize we technically still support `3.7`, but I thought it was fine to change this particular panel to require `3.8`, as `3.7` support is best effort given that it's now EOL'd by the core Python team.

I also removed the relative path location for specific libraries as it was simply blank for me on Python 3.12... I think showing the location of the site packages directory should suffice. If someone later wants to build this out further, they're more than welcome to.

Note that `importlib.metadata.distributions()` does have an outstanding issue that it reports a local editable install twice, but they plan to eventually fix that:
* pypa/setuptools#4170

src/flask_debugtoolbar/panels/versions.py

@@ -17,16 +17,6 @@
 _ = lambda x: x
 
 
-def relpath(location, python_lib):
-    location = os.path.normpath(location)
-    relative = os.path.relpath(location, python_lib)
-    if relative == os.path.curdir:
-        return ''
-    elif relative.startswith(os.path.pardir):
-        return location
-    return relative
-
-
 class VersionDebugPanel(DebugPanel):
     """
     Panel that displays the Flask version.
@@ -48,15 +38,14 @@ def title(self):
 
     def content(self):
         try:
-            import pkg_resources
+            import importlib.metadata
         except ImportError:
             packages = []
         else:
-            packages = sorted(pkg_resources.working_set,
-                              key=lambda p: p.project_name.lower())
+            packages_metadata = [p.metadata for p in importlib.metadata.distributions()]
+            packages = sorted(packages_metadata, key=lambda p: p['Name'].lower())
 
         return self.render('panels/versions.html', {
             'packages': packages,
-            'python_lib': os.path.normpath(get_path('platlib')),
-            'relpath': relpath,
+            'python_lib_dir': os.path.normpath(get_path('platlib')),
         })

src/flask_debugtoolbar/templates/panels/versions.html

@@ -1,32 +1,34 @@
 <h4>Installed Packages</h4>
 
 <p>
-  Installation paths relative to:
+  Current Site Packages Directory:
 </p>
 <pre>
-{{ python_lib }}
+{{ python_lib_dir }}
 </pre>
 
 <table>
   <thead>
     <tr>
       <th>Package</th>
       <th>Version</th>
-      <th>Installed Path</th>
+      <th>Homepage</th>
+      <th>Summary</th>
     </tr>
   </thead>
   <tbody>
     {% for package in packages %}
       <tr class="{{ loop.cycle('flDebugOdd', 'flDebugEven') }}">
-        <td>{{ package.project_name }}</td>
-        <td>{{ package.version }}</td>
-        <td>{{ relpath(package.location, python_lib) }}</td>
+        <td>{{ package.get('Name') }}</td>
+        <td>{{ package.get('Version') }}</td>
+        <td>{{ package.get('Home-page') }}</td>
+        <td>{{ package.get('Summary') }}</td>
       </tr>
     {% else %}
       <tr>
-        <td>setuptools</td>
+        <td>Python 3.8</td>
         <td>NOT INSTALLED</td>
-        <td>Install setuptools to display installed packages and version information</td>
+        <td>This panel requires Python >= 3.8 in order to display installed packages and version information.</td>
       </tr>
     {% endfor %}
   </tbody>