Simplify abi3 usage in pep425tags (#7367)

abi3 refers to the CPython stable ABI, so we should only ensure it
applies in that particular case.

This simplifies the logic in get_platforms() and makes us more
compatible with the behavior of packaging.tags, which only includes abi3
for cpython_tags() and only the literal "abi3".

news/7327.removal

@@ -0,0 +1,2 @@
+Use literal "abi3" for wheel tag on CPython 3.x, to align with PEP 384
+which only defines it for this platform.

src/pip/_internal/pep425tags.py

@@ -10,13 +10,14 @@
 import warnings
 from collections import OrderedDict
 
+from pip._vendor.six import PY2
+
 import pip._internal.utils.glibc
-from pip._internal.utils.compat import get_extension_suffixes
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
     from typing import (
-        Tuple, Callable, List, Optional, Union, Dict, Set
+        Tuple, Callable, List, Optional, Union, Dict
     )
 
     Pep425Tag = Tuple[str, str, str]
@@ -361,12 +362,10 @@ def get_supported(
     if abi:
         abis[0:0] = [abi]
 
-    abi3s = set()  # type: Set[str]
-    for suffix in get_extension_suffixes():
-        if suffix.startswith('.abi'):
-            abi3s.add(suffix.split('.', 2)[1])
+    supports_abi3 = not PY2 and impl == "cp"
 
-    abis.extend(sorted(list(abi3s)))
+    if supports_abi3:
+        abis.append("abi3")
 
     abis.append('none')
 
@@ -419,13 +418,13 @@ def get_supported(
             supported.append(('%s%s' % (impl, versions[0]), abi, arch))
 
     # abi3 modules compatible with older version of Python
-    for version in versions[1:]:
-        # abi3 was introduced in Python 3.2
-        if version in {'31', '30'}:
-            break
-        for abi in abi3s:   # empty set if not Python 3
+    if supports_abi3:
+        for version in versions[1:]:
+            # abi3 was introduced in Python 3.2
+            if version in {'31', '30'}:
+                break
             for arch in arches:
-                supported.append(("%s%s" % (impl, version), abi, arch))
+                supported.append(("%s%s" % (impl, version), "abi3", arch))
 
     # Has binaries, does not use the Python API:
     for arch in arches:

src/pip/_internal/utils/compat.py

@@ -43,7 +43,6 @@
 __all__ = [
     "ipaddress", "uses_pycache", "console_to_str",
     "get_path_uid", "stdlib_pkgs", "WINDOWS", "samefile", "get_terminal_size",
-    "get_extension_suffixes",
 ]
 
 
@@ -189,19 +188,6 @@ def get_path_uid(path):
     return file_uid
 
 
-if PY2:
-    from imp import get_suffixes
-
-    def get_extension_suffixes():
-        return [suffix[0] for suffix in get_suffixes()]
-
-else:
-    from importlib.machinery import EXTENSION_SUFFIXES
-
-    def get_extension_suffixes():
-        return EXTENSION_SUFFIXES
-
-
 def expanduser(path):
     # type: (str) -> str
     """