Fix python loop

[moniika]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

Fixes #4000

Proposed Changes

Fixes error in controls_for code generation when start, end, or increment is not a simple number.

Reason for Changes

This change produces valid python, where before the code generated had syntax error.

Test Coverage

Updated golden file for generator and ran against python interpreter to see that all tests pass.

Manually tested on advanced playground with:

<xml xmlns="https://developers.google.com/blockly/xml">
  <variables>
    <variable>j</variable>
    <variable>i</variable>
    <variable>k</variable>
  </variables>
  <block type="variables_set">
    <field name="VAR">j</field>
    <value name="VALUE">
      <block type="math_number">
        <field name="NUM">0</field>
      </block>
    </value>
    <next>
      <block type="controls_for">
        <field name="VAR">i</field>
        <value name="FROM">
          <shadow type="math_number">
            <field name="NUM">1</field>
          </shadow>
        </value>
        <value name="TO">
          <shadow type="math_number">
            <field name="NUM">10</field>
          </shadow>
          <block type="procedures_callreturn">
            <mutation name="changej"></mutation>
          </block>
        </value>
        <value name="BY">
          <shadow type="math_number">
            <field name="NUM">2</field>
          </shadow>
          <block type="procedures_callreturn">
            <mutation name="changek"></mutation>
          </block>
        </value>
        <statement name="DO">
          <block type="text_print">
            <value name="TEXT">
              <shadow type="text">
                <field name="TEXT">abc</field>
              </shadow>
              <block type="variables_get">
                <field name="VAR">i</field>
              </block>
            </value>
          </block>
        </statement>
      </block>
    </next>
  </block>
  <block type="procedures_defreturn">
    <field name="NAME">changej</field>
    <comment pinned="false" h="80" w="160">Describe this function...</comment>
    <statement name="STACK">
      <block type="math_change">
        <field name="VAR">j</field>
        <value name="DELTA">
          <shadow type="math_number">
            <field name="NUM">1</field>
          </shadow>
        </value>
      </block>
    </statement>
    <value name="RETURN">
      <block type="variables_get">
        <field name="VAR">j</field>
      </block>
    </value>
  </block>
  <block type="procedures_defreturn">
    <field name="NAME">changek</field>
    <comment pinned="false" h="80" w="160">Describe this function...</comment>
    <statement name="STACK">
      <block type="math_change">
        <field name="VAR">k</field>
        <value name="DELTA">
          <shadow type="math_number">
            <field name="NUM">1</field>
          </shadow>
        </value>
      </block>
    </statement>
    <value name="RETURN">
      <block type="variables_get">
        <field name="VAR">k</field>
      </block>
    </value>
  </block>
</xml>