python3Packages.pypandoc: vendor patches, test w/o pandoc-citeproc

* Translate all seds in postPatch into patches (for setting the static
  path and skipping the test that needs network access)
* The patch for the changed pandoc heading generation was simplified:
  Since we know our pandoc version is always that new, we can skip the
  version check.
* Skip the test for pandoc-citeproc: pandoc-citeproc has been deprecated
  in favor of pandoc --citeproc by the upstream pandoc developer.
  pypandoc's testsuite doesn't reflect this yet (although it should
  support --citeproc theoretically) to avoid depending on
  pandoc-citeproc for the checkPhase (as we expect it to break again or
  continue to be broken) we skip the test requiring pandoc-citeproc.

The breakage of pypandoc due to pandoc-citeproc was pointed out here:
https://github.com/NixOS/nixpkgs/pull/116635#issuecomment-809258707
Thank you!

pkgs/development/python-modules/pypandoc/skip-tests.patch

@@ -0,0 +1,20 @@
+diff --git a/tests.py b/tests.py
+index deb50e0..aede281 100755
+--- a/tests.py
++++ b/tests.py
+@@ -179,6 +179,7 @@ class TestPypandoc(unittest.TestCase):
+             received = pypandoc.convert_file(file_url, 'rst')
+             self.assertEqualExceptForNewlineEnd(expected, received)
+ 
++    @unittest.skip("no network access during checkPhase")
+     def test_basic_conversion_from_http_url(self):
+         url = 'https://raw.githubusercontent.com/bebraw/pypandoc/master/README.md'
+         received = pypandoc.convert_file(url, 'html')
+@@ -247,6 +248,7 @@ class TestPypandoc(unittest.TestCase):
+ 
+         self.assertRaises(RuntimeError, f)
+ 
++    @unittest.skip("pandoc-citeproc has been deprecated")
+     def test_conversion_with_citeproc_filter(self):
+         # we just want to get a temp file name, where we can write to
+         filters = ['pandoc-citeproc']