pythongh-81954: Use lazy import warnings in zipfile (python#155326)

Since there is "lazy import warnings" at top level, it's no longer
needed to import warnings explicitly in other places.

Author: vstinner

Lib/zipfile/__init__.py

@@ -620,7 +620,6 @@ def _decodeExtra(self, filename_crc):
                         if up_unicode_name:
                             self.filename = _sanitize_filename(up_unicode_name)
                         else:
-                            import warnings
                             warnings.warn("Empty unicode path extra field (0x7075)", stacklevel=2)
                 except struct.error as e:
                     raise BadZipFile("Corrupt unicode path extra field (0x7075)") from e
@@ -2153,7 +2152,6 @@ def comment(self, comment):
             raise TypeError("comment: expected bytes, got %s" % type(comment).__name__)
         # check for valid comment length
         if len(comment) > ZIP_MAX_COMMENT:
-            import warnings
             warnings.warn('Archive comment is too long; truncating to %d bytes'
                           % ZIP_MAX_COMMENT, stacklevel=2)
             comment = comment[:ZIP_MAX_COMMENT]
@@ -2244,7 +2242,6 @@ def open(self, name, mode="r", pwd=None, *, force_zip64=False):
             if (zinfo._end_offset is not None and
                 zef_file.tell() + zinfo.compress_size > zinfo._end_offset):
                 if zinfo._end_offset == zinfo.header_offset:
-                    import warnings
                     warnings.warn(
                         f"Overlapped entries: {zinfo.orig_filename!r} "
                         f"(possible zip bomb)",
@@ -2494,7 +2491,6 @@ def _extract_member(self, member, targetpath, pwd):
     def _writecheck(self, zinfo):
         """Check for errors before writing a file to the archive."""
         if zinfo.filename in self.NameToInfo:
-            import warnings
             warnings.warn('Duplicate name: %r' % zinfo.filename, stacklevel=3)
         if self.mode not in ('w', 'x', 'a'):
             raise ValueError("write() requires mode 'w', 'x', or 'a'")