This PR contains the following changes for fstring:

fix: so far it was not allowed that a conversion flag is followed by a format specifier.

so far f"{'42'!s:10}" rose a parsing error as a '}' was expected after a conversion flag.This is fix now.

implemented self documenting fstrings in rustpython

I implemented the self documenting fstrings in rustpython, as introduced with python 3.8. I.e., the following syntax is now allowed:

foo="bar"
f"{foo=}"
foo=bar
f"{221=}"
221=42
...

Also combination with conversion flags and format specs are supported.

Open: the behaviour when the type of the expression is a string is so far not conform to cpython, thus those are temporarily modified or skipped.

Ported tests from cpython for self documenting fstrings (and some more for format spec handling)

Imported the tests for selfdocumentign fstrings from cpython to rustpython.