[BUG] walmart.py has ambiguous condition

[GoTTi74]

Expected Behavior

walmart.py should only consider "add to cart" if the item is actually on stock.

Actual Behaviour

On line 69 you have:
if "add to cart" in r.text.lower():

Depending on the item you look up, "r.text" contains a string "add to cart" even if the item is out of stock because the full string is:
<span>Why can’t I add to cart</span>

So, even if an item is out of stock, the condition is met and the script tries to add the item to the cart.

Repro Steps

1. Create a task with an URL that is currently out of stock (e.g. https://www.walmart.com/ip/PlayStation-5-Console/363472942)
2. Start the task
3. PhoenixBot tries to "Add to Basket"

Additional Context

Instead of this code
if "add to cart" in r.text.lower():

I was able to workaround using this because the capitalization is different:
if "Add to cart" in r.text:

The better alternative is to parse the response (r) for "itemprop="availability" and the response should be one of those:

href="//schema.org/OutOfStock"
href="//schema.org/InStock"

So, adding something like this:
availability = doc.xpath('//link[@itemprop="availability"]/@href')[0]

with an appropriate condition might help...