docs: clarify minimum py version

[jo3-l]

Summary
String concatenation is mostly being done with + currently, if we're looking to support >= Python 3.6 we can use f-strings instead which are a really nice addition to the language.

For example:
https://github.com/SWC-Python-Club/python-dl/blob/48909716618a5bcb93ccb95ad250561fcf38f280/source/downloader.py#L21
Could be refactored to

log.info(f'Downloading "{video.title}"', self.stdscr)

Not a big change, obviously, but it's a bit nicer than + in any case.

Proposed solution

• clarify minimum required python version somewhere
• refactor to using f-strings if we're looking to support >= 3.6 only?

[AndreiSva]

Even though I really love the python fstring system, I want this program to run on most python 3 installation. But I will consider switching to str.format() as it offers the basic readability features of fstrings and is more compatible.