Add support for Python 3.8, drop EOL 3.4

[hugovk]

Follow on from #131.

• 3.8 was released on 2019-10-14
• 3.4 was EOL on 2019-03-16

[codecov-io]

Codecov Report

Merging #182 into master will decrease coverage by 0.97%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
- Coverage   86.66%   85.68%   -0.98%     
==========================================
  Files          25       25              
  Lines        4221     4324     +103     
==========================================
+ Hits         3658     3705      +47     
- Misses        563      619      +56

Impacted Files	Coverage Δ
pyasn1/compat/integer.py	96.82% <100%> (-0.1%)	⬇️
pyasn1/codec/ber/encoder.py	84.02% <0%> (-4.97%)	⬇️
pyasn1/codec/ber/decoder.py	86.05% <0%> (-2.2%)	⬇️
pyasn1/type/char.py	87.96% <0%> (-1.35%)	⬇️
pyasn1/type/univ.py	84.39% <0%> (-0.19%)	⬇️
pyasn1/codec/cer/encoder.py	85.61% <0%> (+0.1%)	⬆️
pyasn1/type/base.py	86.29% <0%> (+0.33%)	⬆️
pyasn1/type/useful.py	83.5% <0%> (+0.34%)	⬆️
pyasn1/type/tag.py	89.43% <0%> (+0.44%)	⬆️
pyasn1/type/namedtype.py	85.34% <0%> (+0.66%)	⬆️
... and 3 more

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[etingof]

Thank you!