Fix python_version metadata extraction for bdist_egg distributions

Fixes #1190.

changelog/1192.bug.txt

@@ -0,0 +1 @@
+- Fix python_version metadata extraction for bdist_egg distributions.

tests/test_package.py

@@ -465,4 +465,5 @@ def test_malformed_from_file(monkeypatch):
 
 def test_package_from_egg():
     filename = "tests/fixtures/twine-3.3.0-py3.9.egg"
-    package_file.PackageFile.from_filename(filename, comment=None)
+    p = package_file.PackageFile.from_filename(filename, comment=None)
+    assert p.python_version == "3.9"

twine/package.py

@@ -62,6 +62,21 @@
     ".zip": "sdist",
 }
 
+# Match the regualr expression used by ``pkg_resources``, extended to
+# include the file extension and the end-of-string anchor.
+egg_filename_re = re.compile(
+    r"""
+    (?P<name>[^-]+) (
+        -(?P<ver>[^-]+) (
+            -py(?P<pyver>[^-]+) (
+                -(?P<plat>.+)
+            )?
+        )?
+    )?\.egg$
+    """,
+    re.VERBOSE | re.IGNORECASE,
+)
+
 MetadataValue = Union[Optional[str], Sequence[str], Tuple[str, bytes]]
 
 logger = logging.getLogger(__name__)
@@ -159,8 +174,9 @@ def from_filename(cls, filename: str, comment: Optional[str]) -> "PackageFile":
 
         py_version: Optional[str]
         if dtype == "bdist_egg":
-            (dist,) = importlib_metadata.Distribution.discover(path=[filename])
-            py_version = dist.metadata["Version"]
+            m = egg_filename_re.match(os.path.basename(filename))
+            assert m is not None, "for mypy"
+            py_version = m.group("pyver")
         elif dtype == "bdist_wheel":
             py_version = cast(wheel.Wheel, meta).py_version
         elif dtype == "bdist_wininst":