Drop 2.7 tests and avoid Cython warning

[ajfriend]

Set language level to avoid:

[1/1] Cythonizing /Users/ajfriend/work/h3-py/tests/cython_example.pyx
/Users/ajfriend/work/h3-py/env/lib/python3.11/site-packages/Cython/Compiler/Main.py:381: FutureWarning: Cython directive 'language_level' not set, using '3str' for now (Py3). This has changed from earlier releases! File: /Users/ajfriend/work/h3-py/tests/cython_example.pyx

Also, drop Python 2.7 from test matrix.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ae9865c) 100.00% compared to head (45d1df0) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #324   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines          389       389           
=========================================
  Hits           389       389           

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