Multiple bugs in docs/writing/style.rst: misspelling "falsey", broken "while i in a" example, and stale Python 2 doc URLs

[Sombra-1]

Hi! After reviewing the source files on master, I found three bugs in docs/writing/style.rst (and a few related URLs in other docs files). All are reproducible against the current master branch.

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Bug 1: Misspelling — "falsey" should be "falsy" (style.rst, line ~598)

In the "Check if a variable equals a constant" section, the comment in the "Good" code example reads:

    # or check for the opposite
    if not attr:
        print('attr is falsey!')  # <-- incorrect spelling

The correct Python community term is "falsy" (not "falsey"). This is the spelling used consistently in PEP 8, the Python docs, and throughout the Python community. A quick search confirms "falsey" appears nowhere in the official Python documentation.

Fix: Change 'attr is falsey!' to 'attr is falsy!'

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Bug 2: Broken code example — while i in a: has no i defined (style.rst, "Filtering a list" section)

In the "Filtering a list" section, there are two separate .. code-block:: python blocks both listed under Bad. The second one is:

while i in a:
    a.remove(i)

This block is presented as a standalone example but i is never defined within it. It silently relies on i leaking from the previous for loop. Worse, in the context of the preceding example (a = [3, 4, 5], remove items > 4), after the for loop runs, a = [3, 4] and i = 5 — so the while loop condition 5 in [3, 4] is immediately False and the loop never executes at all. The example fails to demonstrate what it intends to show.

Fix: Give the while loop its own self-contained setup, e.g.:

# Don't make multiple passes through the list
a = [3, 4, 5]
i = 5
while i in a:
    a.remove(i)

Or, more clearly, change i to the literal value it's meant to represent:

while 5 in a:
    a.remove(5)

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Bug 3: Stale / unversioned Python 2 documentation URLs (multiple files)

Several links use the old unversioned http://docs.python.org/... URL format, which redirects to Python 2 documentation. These should be updated to explicitly point to Python 3.

Affected lines in docs/writing/style.rst:

• Line ~575: Truth Value Testing link → http://docs.python.org/library/stdtypes.html#truth-value-testing
• • Line ~637: List comprehensions link → http://docs.python.org/tutorial/datastructures.html#list-comprehensions
• • • Line ~641: Generator expressions link → http://docs.python.org/tutorial/classes.html#generator-expressions
      Also affected:
• docs/writing/structure.rst line ~396: http://docs.python.org/tutorial/modules.html#packages
• • docs/scenarios/cli.rst line ~49: explicit Python 2 link → http://docs.python.org/2/library/argparse.html (in the Plac section)
• • • docs/intro/learning.rst lines ~20 and ~370: unversioned Python tutorial and language reference links
      Fix for all: Replace http://docs.python.org/ with https://docs.python.org/3/ in all instances.

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Happy to submit a PR for any or all of these if that would be helpful!