Use ImmutableDict for Transaction.context

The Transaction.context should be updated only with Transaction.set_context. So using an ImmutableDict will ensure that.

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New review358741002 at https://codereview.tryton.org/358741002/#ps375311002

review358741002 updated at https://codereview.tryton.org/358741002/#ps369561002

New changeset c0f1b12134d8 by Cédric Krier in branch 'default':
Use immutable dictionary for Transaction context
https://hg.tryton.org/trytond/rev/c0f1b12134d8

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closed

New changeset 64b67145a636 by Cédric Krier in branch 'default':
Use immutable dictionary for Transaction context
https://hg.tryton.org/tryton-env/rev/64b67145a636