meas_date is now a datetime.datetime object

[cbrnr]

fixes #42

Starting with MNE 0.20, raw.info["meas_date"] is now a datetime.datetime object. This PR fixes a failing test because of that change.

[codecov]

Codecov Report

Merging #40 into master will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   95.45%   95.39%   -0.07%     
==========================================
  Files           3        3              
  Lines         220      217       -3     
==========================================
- Hits          210      207       -3     
  Misses         10       10              

Impacted Files	Coverage Δ
pybv/tests/test_bv_writer.py	100.00% <100.00%> (ø)

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[sappelhoff]

Thanks @cbrnr