Normalize Windows install instructions for python.org installers and the py launcher

[johnthagen]

• I have added an entry to docs/changelog.md

Summary of changes

Steve Dower reports that 95% of all Windows Python installations use the official python.org installers. By default, this installer installs the py launcher and puts it (only) in the PATH.

• Update pipx Windows installations to use py launcher instructions by default, which will work by default for a vast majority of users
• Remove the unneeded -3 from some py invocations to simplify the instructions. The py launcher will already default to using the latest Python 3 release installed by default and Python 2 has been end of life for almost 4 years

Test plan

1. Install Python from python.org onto a clean Windows machine
2. Run the default pipx installation instructions in this MR
3. Confirm that pipx runs correctly using the default instructions.

[johnthagen]

I wasn't sure if this merited a changelog entry, so I didn't add one. But happy to if requested.

[dukecat0]

Thanks!