python: merge base.json into python.json and change snippets

[REmerald]

It seems that there's no purpose for base.json, there is even duplicate snippets in these 2 files.

So I merged them into python.json.
Also, deleted obsolete (e.g. python2 future and encoding) and useless (e.g. async variants because you can just type async and then use normal def) snippets, added some useful ones, made prefixes more user friendly and shortened descriptions.

[OkelleyDevelopment]

I greatly appreciate this PR (a similar refactor should happen with the LaTeX snippets, but that's a later problem).

Overall I think I'm content with these changes, have you tested these locally too @REmerald ?

[REmerald]

@OkelleyDevelopment, for the most part they do, but there are 2 problems with some neovim setups.

For some reason vim.snippet rejects nested placeholder and placeholder-transform inside a placeholder:

vim.snippet  WARN Failed to parse snippet,
but was able to fix it automatically.
```python
try:
	${1:pass}
except ${2:Exception} as ${3:e}:
	${4:raise $3}
else:
	${5:pass}
```

and inserted text is

try:
    pass
except Exception as e:
    raise 
else:
    pass

Similar situation the nested placeholder like "\t${4:raise ${5:SystemExit}}".

And secondly, for some reason after typing # character(s) the completion menu doesn't show up, only after typing additional letter such as #e the menu appears. This applies to other special characters such as #, / or @.
This means, I can't insert # and ## snippets (/, // and /** as well in C files).
But this can be LazyVim, garymjr/nvim-snippets or nvim-cmp issue, I don't have LuaSnip setup to test.

These 2 issues seem to be related to snippets in almost all languages, not just python, so I think, we can merge this pr and later figure out if it's our issue or not.

[OkelleyDevelopment]

Yeah that seems to be a problem out of scope for this PR, so I'll merge these. FWIW in my local setup (a little different from my last config update), was able to handle this snippet:

"test": {
    "prefix": "nested",
    "body": ["${1:entry ${2:entrytwo}}"]
  }

[OkelleyDevelopment]

Feel free to raise an issue with whatever you've researched (even if it's just a copy-paste from the above text) @REmerald.