[BUG] Fix datetime format

[takos22]

Bug description

CodinGame changed their datetime format, which caused Client.get_pending_clash_of_codes() to error. Described in issue #23.

Checklist

• [ ] If classes/methods/attributes were added/changed then they have been documented and tested.
  • [ ] I have updated the documentation to reflect the changes.
  • [x] I have updated the tests to support the changes.
• [x] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new class, method or attribute).
• [ ] This PR is a breaking change (e.g. classes, methods, attributes or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)
• [x] The code follows the style guidelines and passes linting and testing.

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (ee5d83e) compared to base (a446d96). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #24 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 32 32 Lines 2338 2342 +4 ========================================= + Hits 2338 2342 +4 ``` | [Impacted Files](https://codecov.io/gh/takos22/codingame/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Takos) | Coverage Δ | | |---|---|---| | [codingame/utils.py](https://codecov.io/gh/takos22/codingame/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Takos#diff-Y29kaW5nYW1lL3V0aWxzLnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Takos). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Takos)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.