Fix two bugs in calling heasarc.query_region

[zoghbi-a]

This is fixes two issues:

• Heasarc.query_region(position, catalog=None): This used to fail but the recent addition of _query_execute has pushed the failure down. This fix make the failure happen earlier.

• Passing add_offset=True to Heasarc.query_region fails when columns=None. This was also caused by the recent addition of _query_execute

Tests were added for both cases.

Other minor changes:

• remove the test_tap__maxrec test. Recent changes in pyvo make it irrelevant.
• Style fixes using flake8

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.17%. Comparing base (70c49c0) to head (5af2e5e).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
astroquery/heasarc/core.py	76.92%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3630   +/-   ##
=======================================
  Coverage   73.17%   73.17%           
=======================================
  Files         227      227           
  Lines       21078    21086    +8     
=======================================
+ Hits        15423    15430    +7     
- Misses       5655     5656    +1     

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