Regulate import style in python

[ThreeMonth03]

According to the suggestion in PEP8 and PR #696, the import statements in python files should be regulated. The statements always import module instead of module content, and the import statements use from, which are pythonic.

For example, the following codes meets the regulation. It illustrates module path by from, and it imports module by import.

modmesh/modmesh/pilot/_gui.py

Lines 38 to 39 in a99b8bc

	from . import _pilot_core as _pcore
	from . import airfoil

modmesh/modmesh/onedim/euler1d.py

Lines 11 to 14 in a99b8bc

	try:
	from _modmesh import onedim as _impl # noqa: F401
	except ImportError:
	from .._modmesh import onedim as _impl # noqa: F401

On the contrary, there are some unregulated codes in this project, which should be refactored.
The following code doesn't point out the path using from.

import modmesh.plot.svg as svg

The following code imports module content instead of module (files).

from typing import Any, Callable

from modmesh.testing import TestBase as ModMeshTB

[tigercosmos]

I like this. We should also put this into the style guide document: STYLE.rst.

[yungyuc]

@tigercosmos you just earned yourself the assignment.

[tigercosmos]

My pleasure.

[yungyuc]

Done by #702