Replace ensure with if-statements

[SamWilsn]

What was wrong?

Related to Issue #588

How was it fixed?

https://gist.github.com/SamWilsn/f91192bf4f3988aa5150d5e5d248c2c3

Cute Animal Picture

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 586 lines in your changes are missing coverage. Please review.

Project coverage is 74.28%. Comparing base (f5a3b89) to head (a91977d).
Report is 64 commits behind head on master.

Files	Patch %	Lines
src/ethereum/arrow_glacier/fork.py	0.00%	72 Missing ⚠️
src/ethereum/gray_glacier/fork.py	0.00%	72 Missing ⚠️
src/ethereum/dao_fork/fork.py	0.00%	64 Missing ⚠️
src/ethereum/muir_glacier/fork.py	0.00%	62 Missing ⚠️
src/ethereum/paris/fork.py	48.14%	28 Missing ⚠️
src/ethereum/rlp.py	73.68%	15 Missing ⚠️
src/ethereum/constantinople/fork.py	80.64%	12 Missing ⚠️
src/ethereum/byzantium/fork.py	82.25%	11 Missing ⚠️
src/ethereum/frontier/fork.py	82.25%	11 Missing ⚠️
src/ethereum/homestead/fork.py	82.25%	11 Missing ⚠️
... and 79 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #932      +/-   ##
==========================================
+ Coverage   74.26%   74.28%   +0.02%     
==========================================
  Files         602      643      +41     
  Lines       32670    35932    +3262     
==========================================
+ Hits        24261    26693    +2432     
- Misses       8409     9239     +830     

Flag	Coverage Δ
unittests	74.28% <60.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.