Incorrect detection of self-documenting f-strings

Describe the bug

False-positives are returned for self-documenting f-strings.

To Reproduce

$ cat /tmp/test.py
a = 1
print(f"Hello world={a}")
$ vermin /tmp/test.py
Minimum required versions: 3.8
Incompatible versions:     2

Expected behavior

Expected the minimum required version for this code to be 3.6. A self-documenting f-string has the equals sign on the other side of the variable (i.e. {a=}).

Environment (please complete the following information):

• Vermin version 0.9.2

Thanks in advance!

[bulletmark]

There are still trivial cases tripping this so please re-open. E.g:

$ cat tmp.py
a = 0
b = 0
print(f'a={a} b={b}')

$ vermin -vv --no-tips tmp.py
Detecting python files..
Analyzing using 4 processes..
!2, 3.8      /home/mark/tmp.py
  f-strings require 3.6+
  print(expr) requires 2+ or 3+
  self-documenting fstrings require 3.8+

Minimum required versions: 3.8
Incompatible versions:     2

[gousaiyang]

Actually there are more cases (tested on Python 3.8 on your patch just now):

Case 1 f'{ a = }' Expected: detected as 3.8+ Actual: detected as 3.6+ Notes: ASCII whitespaces ([ \t\n\r\f\v]) are allowed before and after the =, and are preserved in the output.

Case 2 f'{a+b=}', f'{a+1=}' Expected: detected as 3.8+ Actual: detected as 3.6+ Notes: However f'{1+a=}' and f'{1+1=}' are detected correctly.

Case 3 f'{(x:=1)}' Expected: detected as 3.8+ Actual: detected as 3.6+ Notes: This is not a “self-documenting f-string”, this is an f-string containing an assignment expression (which is new in 3.8).

Unfortunately, “self-documenting f-string” or “f-string debugging” is poorly documented in the official documentation (I can only find it in whatsnew and changelog). The PR introducing this change contains some useful information (especially the test cases), along with its corresponding issue (contains relevant discussion when introducing this feature, might be a bit noisy because some previous proposals mentioned were rejected).