Fix/stderr

[itstargetconfirmed]

Hi @aguschin,

I've completed the pull request for #265.

I've implemented a similar fix for MLEM and further details can be found here: iterative/mlem#410

I've completed the following updates:

1. Implemented a context manager stderr_echo to print to stderr.
2. Implemented the printing of Exception and GTOException to stderr as opposed to stdout using the stderr_echo context manager.
3. Updated conftest.py with a Runner class similar to the MLEM project. So that CliRunner instances will be created with standard parameters such as mixed_stderr=False. With this implementation, we will guarantee that the result object will have stderr and stdout properties.
4. Implemented a Runner pytest fixture, similar to the MLEM project, however, no tests are using it currently. I've added it for future use cases.
5. Implemented 2 tests that ensure Exception and GTOException are printed to stderr.
6. Implemented 2 helper functions _check_output_contains(output: str, search_value: str) -> bool: and _check_output_exact_match(output: str, search_value: str) -> bool.
7. The functions in item 6 are used to determine how _check_successful_cmd and _check_failing_cmd should compare stderr and stdout with their expected output.
8. The functions in item 6 are needed because in the current implementation, streams are exacted matched with their expected outputs, however, when printing to stderr the gto application adds the colour red to the output, which prevents exact match. _check_output_contains allows us to match a substring, correcting the issue.

[codecov-commenter]

Codecov Report

Base: 82.79% // Head: 83.56% // Increases project coverage by +0.76% 🎉

Coverage data is based on head (b1c68d5) compared to base (24c165a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #276      +/-   ##
==========================================
+ Coverage   82.79%   83.56%   +0.76%     
==========================================
  Files          17       15       -2     
  Lines        1953     1892      -61     
==========================================
- Hits         1617     1581      -36     
+ Misses        336      311      -25     

Impacted Files	Coverage Δ
gto/cli.py	72.75% <100.00%> (+2.32%)	⬆️
gto/ui.py	85.00% <100.00%> (+1.36%)	⬆️
gto/utils.py
gto/__init__.py
gto/api.py	91.40% <0.00%> (+0.06%)	⬆️

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[aguschin]

LGTM! Thanks a lot @itstargetconfirmed!