[FIX] Feature Constructor: Compatibility with Python 3.8

[lanzagar]

Issue

Feature Constructor did not work on Python 3.8 due to AST changes.

Description of changes

• Add the newly required argument posonlyargs
  As far as I can tell, this does not cause any problems on older versions of python and is silently ignored (always empty anyway).
• Add support for new expression type Constant (which is replacing Num, Str, ...)

[ajdapretnar]

This seems to work fine on Py3.7.

[codecov]

Codecov Report

Merging #4222 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4222      +/-   ##
==========================================
- Coverage   85.95%   85.95%   -0.01%     
==========================================
  Files         394      394              
  Lines       70164    70164              
==========================================
- Hits        60310    60309       -1     
- Misses       9854     9855       +1

[lanzagar]

@ales-erjavec can you check the code in case I missed something before we merge?

[ales-erjavec]

Seems OK.