Moved dateutil to install_requires

Description

I had moved dateutils to extra requires in the previous PR thinking it was only used in tests. Its actually used in datumdef.py. Moving it back to install requires

Fixes #

PR information

[ ] The title of the PR is clear and informative.
[ ] There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
[ ] If applicable, the PR references the bug/issue that it fixes in the description.
[ ] New Unit tests were added for the changes made and CI is passing.

Quality of Code and Contribution Guidelines

[ ] I have read the contribution guidelines.

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.10%. Comparing base (b7fbdca) to head (608d0e7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #1443 +/- ## ======================================= Coverage 85.10% 85.10% ======================================= Files 35 35 Lines 1974 1974 Branches 374 374 ======================================= Hits 1680 1680 Misses 218 218 Partials 76 76 ``` | [Flag](https://app.codecov.io/gh/Azure/azure-functions-python-worker/pull/1443/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/Azure/azure-functions-python-worker/pull/1443/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | `85.05% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure#carryforward-flags-in-the-pull-request-comment) to find out more.

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Azure / azure-functions-python-worker