[FIX] purchase_request: not allow to edit an approved purchase request

[alan196]

This commit fixes the the following issue.

• Create a purchase request with some products and send it to approval
• The manager approves the purchase request.
• After the approval the user can add new products to the request and can abuse

[jarsabot]

    **WARNING**: Could not link to Odoo task. Record ID not found in the title of the MR, supported formats:

{ record type }#{ number }

Available types: 'task#', 't#' (case insensitive)

Examples:

• task#123

• i#123

[jarsabot]

    **WARNING**: Could not link to Odoo task. Record ID not found in the title of the MR, supported formats:

{ record type }#{ number }

Available types: 'task#', 't#' (case insensitive)

Examples:

• task#123

• i#123

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (17.0@b42e398). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             17.0      #20   +/-   ##
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  Coverage        ?   92.85%           
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  Files           ?        4           
  Lines           ?       42           
  Branches        ?        4           
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  Hits            ?       39           
  Misses          ?        0           
  Partials        ?        3           

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