Use packaging.tags.cpython_tags

We assume this function improves the status quo over the current
`_cpython_tags`, so we use it when no arguments need to be customized.

Since `packaging.tags` has its own tests and derives defaults
differently than pep425tags, we also remove the applicable tests that
were testing against the result of a plain call to
`pep425tags.get_supported()`.

src/pip/_internal/pep425tags.py

@@ -11,6 +11,7 @@
 from pip._vendor.packaging.tags import (
     Tag,
     compatible_tags,
+    cpython_tags,
     interpreter_name,
     interpreter_version,
     mac_platforms,
@@ -319,7 +320,10 @@ def _cpython_tags(
     platform=None,  # type: Optional[str]
     abi=None,  # type: Optional[str]
 ):
-    # type: (...) -> List[Tuple[str, str, str]]
+    # type: (...) -> Union[Iterator[Tag], List[Tuple[str, str, str]]]
+    if version is None and platform is None and abi is None:
+        return cpython_tags()
+
     supported = []  # type: List[Tuple[str, str, str]]
 
     # Versions must be given with respect to the preference

tests/unit/test_pep425tags.py

@@ -168,70 +168,8 @@ def test_manylinux_3(self, is_manylinux_compatible):
         assert not is_manylinux_compatible()
 
 
-class TestManylinux1Tags(object):
-
-    @pytest.mark.xfail
-    @patch('pip._internal.pep425tags.is_manylinux2010_compatible',
-           lambda: False)
-    @patch('pip._internal.pep425tags.is_manylinux2014_compatible',
-           lambda: False)
-    @patch('pip._internal.pep425tags.get_platform', lambda: 'linux_x86_64')
-    @patch('pip._internal.utils.glibc.have_compatible_glibc',
-           lambda major, minor: True)
-    @patch('sys.platform', 'linux2')
-    def test_manylinux1_tag_is_first(self):
-        """
-        Test that the more specific tag manylinux1 comes first.
-        """
-        groups = {}
-        for tag in pep425tags.get_supported():
-            groups.setdefault(
-                (tag.interpreter, tag.abi), []
-            ).append(tag.platform)
-
-        for arches in groups.values():
-            if arches == ['any']:
-                continue
-            # Expect the most specific arch first:
-            if len(arches) == 3:
-                assert arches == ['manylinux1_x86_64', 'linux_x86_64', 'any']
-            else:
-                assert arches == ['manylinux1_x86_64', 'linux_x86_64']
-
-
 class TestManylinux2010Tags(object):
 
-    @pytest.mark.xfail
-    @patch('pip._internal.pep425tags.is_manylinux2014_compatible',
-           lambda: False)
-    @patch('pip._internal.pep425tags.get_platform', lambda: 'linux_x86_64')
-    @patch('pip._internal.utils.glibc.have_compatible_glibc',
-           lambda major, minor: True)
-    @patch('sys.platform', 'linux2')
-    def test_manylinux2010_tag_is_first(self):
-        """
-        Test that the more specific tag manylinux2010 comes first.
-        """
-        groups = {}
-        for tag in pep425tags.get_supported():
-            groups.setdefault(
-                (tag.interpreter, tag.abi), []
-            ).append(tag.platform)
-
-        for arches in groups.values():
-            if arches == ['any']:
-                continue
-            # Expect the most specific arch first:
-            if len(arches) == 4:
-                assert arches == ['manylinux2010_x86_64',
-                                  'manylinux1_x86_64',
-                                  'linux_x86_64',
-                                  'any']
-            else:
-                assert arches == ['manylinux2010_x86_64',
-                                  'manylinux1_x86_64',
-                                  'linux_x86_64']
-
     @pytest.mark.parametrize("manylinux2010,manylinux1", [
         ("manylinux2010_x86_64", "manylinux1_x86_64"),
         ("manylinux2010_i686", "manylinux1_i686"),
@@ -255,37 +193,6 @@ def test_manylinux2010_implies_manylinux1(self, manylinux2010, manylinux1):
 
 class TestManylinux2014Tags(object):
 
-    @pytest.mark.xfail
-    @patch('pip._internal.pep425tags.get_platform', lambda: 'linux_x86_64')
-    @patch('pip._internal.utils.glibc.have_compatible_glibc',
-           lambda major, minor: True)
-    @patch('sys.platform', 'linux2')
-    def test_manylinux2014_tag_is_first(self):
-        """
-        Test that the more specific tag manylinux2014 comes first.
-        """
-        groups = {}
-        for tag in pep425tags.get_supported():
-            groups.setdefault(
-                (tag.interpreter, tag.abi), []
-            ).append(tag.platform)
-
-        for arches in groups.values():
-            if arches == ['any']:
-                continue
-            # Expect the most specific arch first:
-            if len(arches) == 5:
-                assert arches == ['manylinux2014_x86_64',
-                                  'manylinux2010_x86_64',
-                                  'manylinux1_x86_64',
-                                  'linux_x86_64',
-                                  'any']
-            else:
-                assert arches == ['manylinux2014_x86_64',
-                                  'manylinux2010_x86_64',
-                                  'manylinux1_x86_64',
-                                  'linux_x86_64']
-
     @pytest.mark.parametrize("manylinuxA,manylinuxB", [
         ("manylinux2014_x86_64", ["manylinux2010_x86_64",
                                   "manylinux1_x86_64"]),