Import underlying parser functions as an underscored variable (#663)

These functions are not a part of the public interface for the modules
they're being imported into.

Co-authored-by: Pradyun Gedam <pradyunsg@users.noreply.github.com>

src/packaging/markers.py

@@ -8,7 +8,14 @@
 import sys
 from typing import Any, Callable, Dict, List, Optional, Tuple, Union
 
-from ._parser import MarkerAtom, MarkerList, Op, Value, Variable, parse_marker
+from ._parser import (
+    MarkerAtom,
+    MarkerList,
+    Op,
+    Value,
+    Variable,
+    parse_marker as _parse_marker,
+)
 from ._tokenizer import ParserSyntaxError
 from .specifiers import InvalidSpecifier, Specifier
 from .utils import canonicalize_name
@@ -189,7 +196,7 @@ def __init__(self, marker: str) -> None:
         #       packaging.requirements.Requirement. If any additional logic is
         #       added here, make sure to mirror/adapt Requirement.
         try:
-            self._markers = _normalize_extra_values(parse_marker(marker))
+            self._markers = _normalize_extra_values(_parse_marker(marker))
             # The attribute `_markers` can be described in terms of a recursive type:
             # MarkerList = List[Union[Tuple[Node, ...], str, MarkerList]]
             #

src/packaging/requirements.py

@@ -5,7 +5,7 @@
 import urllib.parse
 from typing import Any, List, Optional, Set
 
-from ._parser import parse_requirement
+from ._parser import parse_requirement as _parse_requirement
 from ._tokenizer import ParserSyntaxError
 from .markers import Marker, _normalize_extra_values
 from .specifiers import SpecifierSet
@@ -32,7 +32,7 @@ class Requirement:
 
     def __init__(self, requirement_string: str) -> None:
         try:
-            parsed = parse_requirement(requirement_string)
+            parsed = _parse_requirement(requirement_string)
         except ParserSyntaxError as e:
             raise InvalidRequirement(str(e)) from e