Fix tests for Python 4

[hugovk]

We don't yet know if 3.10 or 4.0 will follow Python 3.9, but whichever it is, it will probably happen in 2020 when Python 3.9 reaches beta.

There's some tests which check the Python major version is exactly 3. Instead, check it's at least 3.

Found using https://github.com/asottile/flake8-2020:

$ pip install -U flake8-2020
...
$ flake8 --select YTT
./tests/codec/ber/test_decoder.py:489:120: YTT201: `sys.version_info[0] == 3` referenced (python4), use `>=`
./tests/codec/ber/test_decoder.py:494:91: YTT201: `sys.version_info[0] == 3` referenced (python4), use `>=`
./tests/codec/ber/test_decoder.py:499:83: YTT201: `sys.version_info[0] == 3` referenced (python4), use `>=`
./tests/codec/ber/test_encoder.py:439:56: YTT201: `sys.version_info[0] == 3` referenced (python4), use `>=`

[etingof]

Oh, dear... Time flies...

Thank you!