Use packaging.tags.compatible_tags

We assume this function improves on our existing behavior, so use it
as-is. We will customize the arguments over the next few commits.

Since packaging.tags internally calculates platforms when not provided,
we skip the tests which patch functions assuming that manylinux
compatibility determination depends on them.

src/pip/_internal/pep425tags.py

@@ -10,6 +10,7 @@
 
 from pip._vendor.packaging.tags import (
     Tag,
+    compatible_tags,
     interpreter_name,
     interpreter_version,
     mac_platforms,
@@ -403,7 +404,10 @@ def _compatible_tags(
     platform=None,  # type: Optional[str]
     impl=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 impl is None:
+        return compatible_tags()
+
     supported = []  # type: List[Tuple[str, str, str]]
 
     # Versions must be given with respect to the preference

tests/unit/test_pep425tags.py

@@ -170,6 +170,7 @@ def test_manylinux_3(self, 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',
@@ -200,6 +201,7 @@ def test_manylinux1_tag_is_first(self):
 
 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')
@@ -253,6 +255,7 @@ 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)