false positive no-member after isinstance

[kiefermat]

Steps to reproduce

Given a file a.py:

# pylint: disable=missing-module-docstring

mixed_tuple = (b"a", b"b", b"c", b"d")
byte_tuple = [arg.encode('utf8') if isinstance(arg, str) else arg for arg in mixed_tuple]

for arg in mixed_tuple:
    if isinstance(arg, str):
        print(arg.encode('utf8'))
    else:
        print(arg)

As soon as the mixed_tuple contains at least one string, the no-member warning is no longer shown.

Current behavior

Result of pylint a.py:

************* Module pixy.a
src/pixy/a.py:4:14: E1101: Instance of 'bytes' has no 'encode' member (no-member)
src/pixy/a.py:8:14: E1101: Instance of 'bytes' has no 'encode' member (no-member)

---------------------------------------------------------------------
Your code has been rated at -6.67/10 (previous run: -15.00/10, +8.33)

Expected behavior

No warning should be shown

pylint --version output

Result of pylint --version output:

pylint 2.7.2
astroid 2.5.1
Python 3.9.1

This behaviour has been introduced with 2.7.0