use new asyncio.all_tasks if available

[dirk-thomas]

To avoid deprecation warning in Python 3.7+:

colcon_core\executor\sequential.py:94: PendingDeprecationWarning: Task.all_tasks() is deprecated, use asyncio.all_tasks() instead

[codecov-io]

Codecov Report

Merging #187 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
+ Coverage   78.03%   78.06%   +0.02%     
==========================================
  Files          53       53              
  Lines        3051     3055       +4     
  Branches      511      511              
==========================================
+ Hits         2381     2385       +4     
  Misses        642      642              
  Partials       28       28

Impacted Files	Coverage Δ
colcon_core/executor/sequential.py	95.65% <100%> (+0.26%)	⬆️

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[cottsay]

This should clean up the warnings coming out of the Fedora rawhide RPM builds for colcon.