(core) Improve encoding detection for csv imports, and make encoding an editable option.

Summary:
- Using a sample of data was causing poor detection if the sample were
  cut mid-character. Switch to using line-based detection.
- Add a simple option for changing encoding. No convenient UI is offered
  since config UI is auto-generated, but this at least makes it possible to
  recover from bad guesses.
- Upgrades chardet library for good measure.

- Also fixes python3-building step, to more reliably rebuild Python
  dependencies when requirements3.* files change.

Test Plan:
Added a python-side test case, and a browser test that encodings can
be switched, errors are displayed, and wrong encodings fail recoverably.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3979

sandbox/requirements3.txt

@@ -10,7 +10,7 @@ asttokens==2.2.1
     # via
     #   friendly-traceback
     #   stack-data
-chardet==4.0.0
+chardet==5.1.0
     # via -r core/sandbox/requirements3.in
 et-xmlfile==1.0.1
     # via openpyxl