perf: cache requires-python checks & skip debug logging

The `requires-python` check is pretty fast, but when performed for
10000 links, the checks consume a nontrival amount of time. For example,
while installing (pre-cached + --dry-run) a pared down list of
homeassistant dependencies (117 in total), link evaluation took 15% of
the total runtime, with  check_requires_python() accounting for half
(7.5%) of that.

The cache can be kept pretty small as `requires-python` specifiers often
repeat, and when they do change, it's often in chunks (or between
entirely different packages). For example, setuptools has like 1500
links, but only ~30 different `requires-python` specifiers.

In addition, _log_skipped_link() is a hot method and unfortunately
expensive as it hashes the link on every call. Fortunately, we can
return early when debug logging is not enabled. In the same
homeassistant run, this saves 0.7% of the runtime.

news/13128.feature.rst

@@ -0,0 +1 @@
+Cache ``python-requires`` checks while filtering potential installation candidates.

src/pip/_internal/index/package_finder.py

@@ -732,6 +732,11 @@ def _sort_links(self, links: Iterable[Link]) -> List[Link]:
         return no_eggs + eggs
 
     def _log_skipped_link(self, link: Link, result: LinkType, detail: str) -> None:
+        # This is a hot method so don't waste time hashing links unless we're
+        # actually going to log 'em.
+        if not logger.isEnabledFor(logging.DEBUG):
+            return
+
         entry = (link, result, detail)
         if entry not in self._logged_links:
             # Put the link at the end so the reason is more visible and because

src/pip/_internal/utils/packaging.py

@@ -11,6 +11,7 @@
 logger = logging.getLogger(__name__)
 
 
+@functools.lru_cache(maxsize=96)
 def check_requires_python(
     requires_python: Optional[str], version_info: Tuple[int, ...]
 ) -> bool: