MOL2 writer does not work on universes

[IAlibay]

Expected behavior

This is related to: #2110 (comment)

As per its docstring, the MOL2Writer should take in either an atomgroup or a universe.

Actual behavior

Passing a universe causes a failure in encode_block when calling bondgroup = obj.bonds.atomgroup_intersection(obj, strict=True).

Code to reproduce the behavior

import MDAnalysis as mda
from MDAnalysis.tests.datafiles import mol2_comments_header
from MDAnalysis.coordinates.MOL2 import MOL2Writer

u = mda.Universe(mol2_comments_header)

with MOL2Writer("test.mol2", n_atoms=9) as w:
    for i, ts in enumerate(u.trajectory):
        w.write(u)

Current version of MDAnalysis

• Which version are you using? (run python -c "import MDAnalysis as mda; print(mda.__version__)") 0.20.2-dev0
• Which version of Python (python -V)? 3.7.7
• Which operating system? Ubuntu 20.04 LTS

[IAlibay]

As pointed out by @lilyminium the fix here should be trivial enough: #2110 (comment)