pyproj.crs: add imports to __all__

[adamjstewart]

This PR explicitly exports all classes in pyproj.crs, allowing type checkers like mypy to better use the type hints. Without this change, mypy will complain with an error like:

> mypy --strict foo.py
foo.py:3: error: Module "pyproj.crs" does not explicitly export attribute "CRS"  [attr-defined]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.56%. Comparing base (6a38287) to head (1262ed6).
Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1494      +/-   ##
==========================================
+ Coverage   96.50%   96.56%   +0.06%     
==========================================
  Files          20       20              
  Lines        1830     1833       +3     
==========================================
+ Hits         1766     1770       +4     
+ Misses         64       63       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[adamjstewart]

@snowman2 can you review?

[snowman2]

Thanks @adamjstewart 👍