enhancement: add new backend to load and dump python code as data

[ssato]

This PR adds new backend module consists of the code to

• load data from python code which contains a literal data represents a very basic and simple data
• load data from python code which contains a data may be constructed with code run
• dump data to a python code

along with docs and test cases and data.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (74c061b) 95.87% compared to head (898c148) 95.91%.

Files	Patch %	Lines
src/anyconfig/backend/python/loader.py	92.59%	2 Missing :warning:
src/anyconfig/backend/python/utils.py	96.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## next #161 +/- ## ========================================== + Coverage 95.87% 95.91% +0.03% ========================================== Files 271 277 +6 Lines 4630 4744 +114 ========================================== + Hits 4439 4550 +111 - Misses 191 194 +3 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[ssato]

This also may close #138