Rename 'async' property to fix SyntaxError with Python 3.7.

[sebastic]

Overview

Rename async property to asynchronous to fix SyntaxError with Python 3.7.

async is now a reserved keyword in Python 3.7.

Related Issue / Discussion

Fixes: #570

Additional Information

Backwards incompatible syntax changes:

• async and await are now reserved keywords.

https://docs.python.org/3/whatsnew/3.7.html

Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines

[codecov]

Codecov Report

Merging #571 into master will not change coverage.
The diff coverage is 16.66%.

@@           Coverage Diff           @@
##           master     #571   +/-   ##
=======================================
  Coverage   55.81%   55.81%           
=======================================
  Files          28       28           
  Lines        6355     6355           
  Branches     1349     1349           
=======================================
  Hits         3547     3547           
- Misses       2422     2423    +1     
+ Partials      386      385    -1

Flag	Coverage Δ
#integrationtests	54.46% <16.66%> (ø)	⬆️
#unittests	7.58% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pycsw/ogc/csw/csw3.py	47.5% <0%> (ø)	⬆️
pycsw/ogc/csw/csw2.py	71.73% <0%> (ø)	⬆️
pycsw/server.py	64.35% <25%> (ø)	⬆️
pycsw/ogc/fes/fes2.py	39.13% <0%> (ø)	⬆️

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