gh-89739: gh-77140: Support zip64 in zipimport (GH-94146)

* Reads zip64 files as produced by the zipfile module
* Include tests (somewhat slow, however, because of the need to create "large" zips)
* About the same amount of strictness reading invalid zip files as zipfile has
* Still works on files with prepended data (like pex)

There are a lot more test cases at https://github.com/thatch/zipimport64/ that give me confidence that this works for real-world files.

Fixes #89739 and #77140.

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Co-authored-by: Itamar Ostricher <itamarost@gmail.com>
Reviewed-by: Gregory P. Smith <greg@krypto.org>

Lib/importlib/_bootstrap_external.py

@@ -81,6 +81,11 @@ def _pack_uint32(x):
     return (int(x) & 0xFFFFFFFF).to_bytes(4, 'little')
 
 
+def _unpack_uint64(data):
+    """Convert 8 bytes in little-endian to an integer."""
+    assert len(data) == 8
+    return int.from_bytes(data, 'little')
+
 def _unpack_uint32(data):
     """Convert 4 bytes in little-endian to an integer."""
     assert len(data) == 4