* Constrain `icontract` version to <2.4.0

Backwards-compatible bug fix.

* Add Python version 3.7, 3.8 to test matrix

Note: 3.5, 3.6 currently fail with:
```
YAPF'ing...
Mypy'ing...
Success: no issues found in 3 source files
Pyicontract-lint'ing...
tests/test_sphinx_icontract.py: 'ascii' codec can't decode byte 0xe2 in position 991: ordinal not in range(128) (unreadable)
sphinx_icontract/__init__.py: 'ascii' codec can't decode byte 0xe2 in position 3580: ordinal not in range(128) (unreadable)
[REDACTED]
subprocess.CalledProcessError: Command '['pyicontract-lint', 'tests', 'sphinx_icontract']' returned non-zero exit status 1.
```

* Fixed compatibility with `icontract` >=2.4.0

Breaking changes made in Parquery/icontract#178.

• Commit e414fa0
• Compare 2bd9f95..e414fa0
• Branch master

Marko Ristin