Set mtime on setup.py/PKG-INFO when building sdist

Previously, the mtime was the default value of 0.

Fixes #670.

This stops the following warning when using gnu tar:
implausibly old time stamp 1970-01-01 01:00:00

poetry/masonry/builders/sdist.py

@@ -2,6 +2,7 @@
 import os
 import re
 import tarfile
+import time
 
 from collections import defaultdict
 from copy import copy
@@ -83,12 +84,14 @@ class SdistBuilder(Builder):
             setup = self.build_setup()
             tar_info = tarfile.TarInfo(pjoin(tar_dir, "setup.py"))
             tar_info.size = len(setup)
+            tar_info.mtime = time.time()
             tar.addfile(tar_info, BytesIO(setup))
 
             pkg_info = self.build_pkg_info()
 
             tar_info = tarfile.TarInfo(pjoin(tar_dir, "PKG-INFO"))
             tar_info.size = len(pkg_info)
+            tar_info.mtime = time.time()
             tar.addfile(tar_info, BytesIO(pkg_info))
         finally:
             tar.close()

tests/masonry/builders/test_sdist.py

@@ -408,6 +408,9 @@ def test_default_with_excluded_data(mocker):
         assert "my-package-1.2.3/pyproject.toml" in names
         assert "my-package-1.2.3/setup.py" in names
         assert "my-package-1.2.3/PKG-INFO" in names
+        # all last modified times should be set to a valid timestamp
+        for tarinfo in tar.getmembers():
+            assert 0 < tarinfo.mtime
 
 
 def test_proper_python_requires_if_two_digits_precision_version_specified():