Fix pandas datetime formatting

[AsafMah]

Fixed

• Pandas now correctly parses all dates #514

Changes in the test file:

• Removing the test class (not needed in pytest)
• Removing the test optionality (dev_dependencies always installs pandas)
• Added new test

[github-actions[bot]]

Test Results

       6 files  ±0         6 suites  ±0   21m 26s :stopwatch: - 2m 21s    295 tests +1     274 :heavy_check_mark: +1    21 :zzz: ±0  0 :x: ±0  1 770 runs  +6  1 644 :heavy_check_mark: +6  126 :zzz: ±0  0 :x: ±0 

Results for commit 69bdfb85. ± Comparison against base commit 882dd00e.

This pull request removes 1 and adds 2 tests. Note that renamed tests count towards both.

``` azure-kusto-data.tests.test_helpers.TestDataFrameFromResultsTable ‑ test_dataframe_from_result_table ``` ``` azure-kusto-data.tests.test_helpers ‑ test_dataframe_from_result_table azure-kusto-data.tests.test_helpers ‑ test_pandas_mixed_date ```

:recycle: This comment has been updated with latest results.

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (882dd00) 88.96% compared to head (69bdfb8) 88.92%.

Files	Patch %	Lines
azure-kusto-data/azure/kusto/data/helpers.py	71.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #515 +/- ## ========================================== - Coverage 88.96% 88.92% -0.04% ========================================== Files 44 44 Lines 3696 3702 +6 ========================================== + Hits 3288 3292 +4 - Misses 408 410 +2 ```

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