Fix cgi and importlib_resources deprecations

[hugovk]

gidgethub/sansio.py:8: 10 warnings
  /private/tmp/gidgethub/gidgethub/sansio.py:8: DeprecationWarning: 'cgi' is deprecated and slated for removal in Python 3.13
    import cgi

Fix using email module as suggested at https://peps.python.org/pep-0594/#cgi

---

tests/test_abc.py: 10 warnings
  /private/tmp/gidgethub/tests/test_abc.py:705: DeprecationWarning: read_binary is deprecated. Use files() instead. Refer to https://importlib-resources.readthedocs.io/en/latest/using.html#migrating-from-legacy for migration advice.
    payload = importlib_resources.read_binary(graphql_samples, payload_filename)

tests/test_apps.py::TestGitHubAppUtils::test_get_jwt
  /private/tmp/gidgethub/tests/test_apps.py:24: DeprecationWarning: read_binary is deprecated. Use files() instead. Refer to https://importlib-resources.readthedocs.io/en/latest/using.html#migrating-from-legacy for migration advice.
    private_key = importlib_resources.read_binary(rsa_key_samples, "test_rsa_key")

tests/test_apps.py::TestGitHubAppUtils::test_get_installation_access_token
  /private/tmp/gidgethub/tests/test_apps.py:41: DeprecationWarning: read_binary is deprecated. Use files() instead. Refer to https://importlib-resources.readthedocs.io/en/latest/using.html#migrating-from-legacy for migration advice.
    private_key = importlib_resources.read_binary(rsa_key_samples, "test_rsa_key")

Fix using files() API as suggested at https://importlib-resources.readthedocs.io/en/latest/using.html#migrating-from-legacy

[codecov-commenter]

Codecov Report

Merging #185 (24ab9d4) into main (8c60e56) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #185   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          461       463    +2     
  Branches       102       108    +6     
=========================================
+ Hits           461       463    +2     

Impacted Files	Coverage Δ
gidgethub/sansio.py	100.00% <100.00%> (ø)

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[brettcannon]

Looks like mypy doesn't like the typing of the change.

[hugovk]

How's this?

[brettcannon]

Thanks!