Don't believe there is code to do that.  You will probably need to write some code to handle the special formatting you require.

How about trying this piece of coding based on the text of your question/answer:

<t t-set="mod_qty">
    formatLang(l.product_uom_qty, digit=0)
</t>

<t t-if="mod_qty == l.product_uom_qty">
    <t t-esc="mod_qty" />
</t>

<t t-if="mod_qty != l.product_uom_qty">
    <t t-esc="(l.product_uom_qty * 1)" />
</t>

Or this generic code that will print your list of numbers.

<t t-foreach="[3.000, 3.050, 3.005, 3.0005]" t-as="my_qty">
    <t t-set="mod_qty">
        formatLang(my_qty, digit=0)
    </t>

    <t t-if="mod_qty == my_qty">
        <p><t t-esc="mod_qty"/></p>
    </t>

    <t t-if="mod_qty != my_qty">
        <p><t t-esc="(my_qty * 1)"/></p>
    </t>
</t>

I've gotten closer.

https://www.purekarting.com/odoo/sale.report_saleorder_closer.pdf

By using <t t-esc="(l.product_uom_qty * 1)" />, it converts the qty from a string to a number.  Trailing zeros past the first decimal precision are dropped in numbers.

So, seeing as how I have decimal precision set to 3 on product_uom_qty, values will be reported as:

3.000 prints as 3.0
3.050 prints as 3.05
3.005 prints as 3.005
3.0005 is rounded up by Odoo to 3.001, and then prints as 3.001

Close but not perfect.  I'd like to drop that trailing .0 from the whole number quantities still.

what you could  do is :

import openerp.addons.decimal_precision as dp

and also  for example this will allow your field to acces the decimal accuary of account so if you set account to have 8 decimal places your field will have 8 decimal places:

'rent':fields.float('Rent',digits_compute= dp.get_precision('Account')),