urllib3: upgrade to allowed_methods

[dhalperi]

See: https://urllib3.readthedocs.io/en/stable/reference/urllib3.util.html

[batfish-bot]

This change is 

[codecov[bot]]

Codecov Report

Merging #882 (adae5dc) into master (26aee45) will not change coverage. The diff coverage is n/a.

:exclamation: Current head adae5dc differs from pull request most recent head 3eafc7c. Consider uploading reports for the commit 3eafc7c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #882 +/- ## ======================================= Coverage 85.99% 85.99% ======================================= Files 30 30 Lines 3992 3992 Branches 665 665 ======================================= Hits 3433 3433 Misses 412 412 Partials 147 147 ``` | [Impacted Files](https://app.codecov.io/gh/batfish/pybatfish/pull/882?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=batfish) | Coverage Δ | | |---|---|---| | [pybatfish/client/\_diagnostics.py](https://app.codecov.io/gh/batfish/pybatfish/pull/882?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=batfish#diff-cHliYXRmaXNoL2NsaWVudC9fZGlhZ25vc3RpY3MucHk=) | `74.31% <ø> (ø)` | | | [pybatfish/client/resthelper.py](https://app.codecov.io/gh/batfish/pybatfish/pull/882?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=batfish#diff-cHliYXRmaXNoL2NsaWVudC9yZXN0aGVscGVyLnB5) | `47.72% <ø> (ø)` | | | [pybatfish/client/restv2helper.py](https://app.codecov.io/gh/batfish/pybatfish/pull/882?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=batfish#diff-cHliYXRmaXNoL2NsaWVudC9yZXN0djJoZWxwZXIucHk=) | `70.52% <ø> (ø)` | |