Port generate-encoding-data.py to Python 3

[Nopey]

No changes to the generated files, as those still generate the same.

[Nopey]

Drafting while I investigate what's going on in the generated codepage/{lib,test}.rs, which wants to add two encodings

+    assert_eq!(to_encoding(10007), Some(X_MAC_CYRILLIC));
+    assert_eq!(to_encoding(21010), Some(UTF_16LE));

EDIT: These seem to be correct, which would just make the codepage crate's source code out of date. Undrafting. Commit 11b0b46b3ca7a0df57ad7c5816b062518417e7d2 adds them to the python script:

  10007: "x-mac-cyrillic",
  21010: "UTF-16LE", # Undocumented; needed by calamine for Excel compat

EDIT 2: These two's absence is issue #70.