Failing spec for standard Python library updates

python/spec/dependabot/python/update_checker_spec.rb

@@ -532,7 +532,7 @@
       its([:requirement]) { is_expected.to eq("==2.6.0") }
     end
 
-    context "when there is a pyproject.toml file" do
+    context "when there is a pyproject.toml file with poetry dependencies" do
       let(:dependency_files) { [requirements_file, pyproject] }
       let(:pyproject_fixture_name) { "caret_version.toml" }
 
@@ -577,6 +577,51 @@
       end
     end
 
+    context "when there is a pyproject.toml file with pep621 dependencies" do
+      let(:dependency_files) { [pyproject] }
+      let(:pyproject_fixture_name) { "standard_python_caret_version.toml" }
+
+      let(:dependency) do
+        Dependabot::Dependency.new(
+          name: "requests",
+          version: "1.2.3",
+          requirements: [{
+            file: "pyproject.toml",
+            requirement: "~=1.0.0",
+            groups: [],
+            source: nil
+          }],
+          package_manager: "pip"
+        )
+      end
+
+      let(:pypi_url) { "https://pypi.org/simple/requests/" }
+      let(:pypi_response) do
+        fixture("pypi", "pypi_simple_response_requests.html")
+      end
+
+      context "for a library" do
+        before do
+          stub_request(:get, "https://pypi.org/pypi/pendulum/json/").
+            to_return(
+              status: 200,
+              body: fixture("pypi", "pypi_response_pendulum.json")
+            )
+        end
+
+        its([:requirement]) { is_expected.to eq(">=1,<3") }
+      end
+
+      context "for a non-library" do
+        before do
+          stub_request(:get, "https://pypi.org/pypi/pendulum/json/").
+            to_return(status: 404)
+        end
+
+        its([:requirement]) { is_expected.to eq("~=2.19.1") }
+      end
+    end
+
     context "when there were multiple requirements" do
       let(:dependency) do
         Dependabot::Dependency.new(

python/spec/fixtures/pyproject_files/standard_python_caret_version.toml

@@ -0,0 +1,13 @@
+[project]
+name = "pendulum"
+version = "2.0.0"
+homepage = "https://github.com/roghu/py3_projects"
+license = "MIT"
+readme = "README.md"
+authors = ["Dependabot <support@dependabot.com>"]
+description = "Python datetimes made easy"
+
+dependencies = [
+  "python~=3.7",
+  "requests~=1.0.0"
+]