(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

app/client/components/ParseOptions.ts

@@ -1,6 +1,7 @@
 import {bigBasicButton, bigPrimaryButton} from 'app/client/ui2018/buttons';
 import {squareCheckbox} from 'app/client/ui2018/checkbox';
 import {testId, theme} from 'app/client/ui2018/cssVars';
+import {makeLinks} from 'app/client/ui2018/links';
 import {cssModalButtons} from 'app/client/ui2018/modals';
 import {ParseOptionSchema} from 'app/plugin/FileParserAPI';
 import {Computed, dom, DomContents, IDisposableOwner, input, Observable, styled} from 'grainjs';
@@ -60,7 +61,7 @@ export function buildParseOptionsForm(
   return [
     cssParseOptionForm(
       items.map((item) => cssParseOption(
-        cssParseOptionName(item.label),
+        cssParseOptionName(makeLinks(item.label)),
         optionToInput(owner, item.type, optionsMap.get(item.name)!),
         testId('parseopts-opt'),
       )),