Fix hpx interface

[omodei]

fix interface to new gammapy

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.

Project coverage is 50.67%. Comparing base (00a7fc5) to head (a59af22).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
fermipy/plotting.py	16.66%	5 Missing ⚠️
fermipy/gtanalysis.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #608      +/-   ##
==========================================
+ Coverage   49.65%   50.67%   +1.02%     
==========================================
  Files         143      143              
  Lines       27191    27196       +5     
==========================================
+ Hits        13502    13782     +280     
+ Misses      13689    13414     -275     

Flag	Coverage Δ
unittests	50.67% <53.84%> (+1.02%)	⬆️

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[ndilalla]

@omodei I updated the requirements relaxing the scipy and astropy dependencies and added a workflow to test against the development version of fermitools. Can you take a quick look? If you are okay, feel free to merge it!