Update Minimum Required Requests Version to 2.25.0

[btstevens89]

With https://github.com/zach-taylor/splunk_handler/pull/49, the underlying urllib3 minimum version required increased to 1.26.0. Requests 2.25.0 is the first version that pulls in urllib3 1.26.0.

urllib3 1.26.0 (2020-11-10) https://urllib3.readthedocs.io/en/latest/changelog.html#id6

Deprecated Retry options Retry.DEFAULT_METHOD_WHITELIST, Retry.DEFAULT_REDIRECT_HEADERS_BLACKLIST and 
Retry(method_whitelist=...) in favor of Retry.DEFAULT_ALLOWED_METHODS, Retry.DEFAULT_REMOVE_HEADERS_ON_REDIRECT,
and Retry(allowed_methods=...) (Pull #2000) Starting in urllib3 v2.0: Deprecated options will be removed

Requests 2.25.0 (2020-11-11) https://docs.python-requests.org/en/latest/community/updates/#id3

Dependencies
    Requests now supports urllib3 v1.26.