get_python_lib() returns path to python3 instead of python3.6

[Alexander]

Python version is Python 3.6.7 (default, Oct 22 2018, 11:32:17)
Ubuntu 18.04.01

When I call get_python_lib() function to get the Python lib path to install a module it returns '/usr/lib/python3/dist-packages' instead of python3.6 dir:
python3 -c "from distutils.sysconfig import *;print(get_python_lib())"

This is because the source code '/usr/lib/python3.6/distutils/sysconfig.py' has the branch on Ubuntu:

Code:
------------
    if os.name == "posix":
        libpython = os.path.join(prefix,
                                 "lib", "python" + get_python_version())
        if standard_lib:
            return libpython
        elif (is_default_prefix and
              'PYTHONUSERBASE' not in os.environ and
              'VIRTUAL_ENV' not in os.environ and
              'real_prefix' not in sys.__dict__ and
              sys.prefix == sys.base_prefix):
            return os.path.join(prefix, "lib", "python3", "dist-packages")
------------

However, this code does not present in the original cpython code 3.6.7:
https://github.com/python/cpython/blob/ca7d2933a388677cc3bbc621913b479452c0f25a/Lib/distutils/sysconfig.py#L109

Is it the right behavior?

Why the get_python_lib() result should depend on PYTHONUSERBASE and VIRTUAL_ENV and points to python3 instead of python3.6

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: python3 3.6.7-1~18.04
ProcVersionSignature: Ubuntu 4.15.0-44.47-generic 4.15.18
Uname: Linux 4.15.0-44-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.3
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Tue Feb 5 12:41:28 2019
InstallationDate: Installed on 2018-09-06 (151 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: python3-defaults
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[vital (vsbogd)]

The issue is that if `get_python_lib()` is called with `prefix='/usr/local'` argument then Ubuntu version returns:
``` sh
$ /usr/bin/python3 -c "from distutils.sysconfig import *;print(get_python_lib(prefix='/usr/local'))"
/usr/local/lib/python3/dist-packages
```

while mainstream Python returns:
```sh
$ python3 -c "from distutils.sysconfig import *;print(get_python_lib(prefix='/usr/local'))"
/usr/local/lib/python3.6/site-packages
```

`sys.path` (in both Ubuntu and mainstream Python) doesn't contain `/usr/local/lib/python3/dist-packages` path, it contains `/usr/local/lib/python3.6/dist-packages` instead, so when `/usr/local` prefix is used things don't work:
```
$ PYTHONPATH='' /usr/bin/python3 -c "import sys; print(sys.path)"
['', '/usr/lib/python36.zip', '/usr/lib/python3.6', '/usr/lib/python3.6/lib-dynload', '/home/vital/.local/lib/python3.6/site-packages', '/usr/local/lib/python3.6/dist-packages', '/usr/lib/python3/dist-packages']
```

[vital (vsbogd)]

Reported this to the correct package: https://bugs.launchpad.net/ubuntu/+source/python3-stdlib-extensions/+bug/1832215