Drop 'encoding' keyword when loading json files, removed in Python 3.8

[michaelweghorn]

Otherwise, running tests during the build with Python 3.9
fails like this:

----------------------------------------------------------
Testing testSuiteSelectors
----------------------------------------------------------
Traceback (most recent call last):
  File "/<<PKGBUILDDIR>>/test/./test_lib_apt_repos.py", line 326, in <module>
    main()
  File "/<<PKGBUILDDIR>>/test/./test_lib_apt_repos.py", line 320, in main
    globals()[method]()
  File "/<<PKGBUILDDIR>>/test/./test_lib_apt_repos.py", line 49, in testSuiteSelectors
    dumpSelectedSuites(apt_repos.getSuites(selector), selector)
  File "/<<PKGBUILDDIR>>/test/../apt_repos/__init__.py", line 212, in getSuites
    jsonData = json.load(file, encoding="utf-8")
  File "/usr/lib/python3.9/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib/python3.9/json/__init__.py", line 359, in loads
    return cls(**kw).decode(s)
TypeError: __init__() got an unexpected keyword argument 'encoding'

It looks like the encoding keyword has been removed in newer Python
versions.

The documentation for json.load says [1]:

Changed in version 3.6: fp can now be a binary file.
The input encoding should be UTF-8, UTF-16 or UTF-32.

The doc for Python 3.7 json.loads method (i.e. an extra 's' at the
end of the method name) says:

The other arguments have the same meaning as in load(), except encoding
which is ignored and deprecated.

and for 3.8, the 'encoding' param for that method is gone.

This reverts commit e63e49d,
"using utf-8 encoding when loading json files".

[1] https://docs.python.org/3/library/json.html

[michaelweghorn]

@chrlutz Any thoughts on this PR, #8 and #9?