benchmark: analyze and py36-friendliness

[tiborsimko]

The benchmark script is being run on CentOS7/CentOS8 boxes where python3 version is still 3.6.

All commands work except for analyze:

$ python reana_bench.py analyze -w susyewkfullhad-boosted-nosys-run-10
...
File "/reana_bench.py", line 433, in analyze
  processed_results = _derive_metrics(original_results)
File "/reana_bench.py", line 223, in _derive_metrics
  df = _pre_process_results(df)
File "/reana_bench.py", line 215, in _pre_process_results
  df["started"] = _convert_str_date_to_epoch(df["started"])
File "/reana_bench.py", line 181, in _convert_str_date_to_epoch
  lambda x: int(time.mktime(datetime.fromisoformat(x).timetuple()))
File "/_venv/lib64/python3.6/site-packages/pandas/core/series.py", line 4213, in apply
  mapped = lib.map_infer(values, f, convert=convert_dtype)
File "pandas/_libs/lib.pyx", line 2403, in pandas._libs.lib.map_infer
File "/reana_bench.py", line 181, in <lambda>
  lambda x: int(time.mktime(datetime.fromisoformat(x).timetuple()))
AttributeError: type object 'datetime.datetime' has no attribute 'fromisoformat'
$ python -V
Python 3.6.8

It would be good to amend the script to make it py36 friendly so that all the benchmarking steps will be runnable.