docs: Note that SnakeViz is Python 3.7+

[matthewfeickert]

• Note that SnakeViz is supported on Python 3.7+.
• Note the last version of SnakeViz to support Python 2 is v2.1.2.
• Note that Python 2 is EOL.

I also stuck in an opinion here of

• Advocate for python -m pip pattern which is the only way to be truly sure that the pip being used is the pip for the Python runtime you want.

[matthewfeickert]

@jiffyclub this is ready for review.

[matthewfeickert]

@jiffyclub gentle ping on this.

[jiffyclub]

Thanks!