Always close stderr after subprocess completion in call_subprocess()

When running Python with warnings enabled, fixes warnings of the form:

    .../site-packages/pip/_internal/vcs/versioncontrol.py:773: ResourceWarning: unclosed file <_io.BufferedReader name=12>
      return call_subprocess(cmd, cwd,

news/9156.bugfix.rst

@@ -0,0 +1 @@
+Fix ResourceWarning in VCS subprocesses

src/pip/_internal/vcs/versioncontrol.py

@@ -150,6 +150,8 @@ def call_subprocess(
     finally:
         if proc.stdout:
             proc.stdout.close()
+        if proc.stderr:
+            proc.stderr.close()
 
     proc_had_error = (
         proc.returncode and proc.returncode not in extra_ok_returncodes