Be more strict about url scheme parsing

[vanschelven]

Pull Request check-list

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• Does $ tox pass with this change (including linting)?
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Description of change

The error message implied that urls had to start with scheme://.
However, if the double slash was left out, the url parsed just fine
and the part that was ostensibly intended to be the hostname ended
up as part of the path, whereas the default (localhost) would be
used for the hostname. This commit makes the check as strict as the
error message implies by including a check for the double slash.

[vanschelven]

I think the pipeline troubles are independent of this fix (see #2344)

[codecov-commenter]

Codecov Report

Merging #2343 (4b6d68e) into master (031b208) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2343      +/-   ##
==========================================
+ Coverage   92.19%   92.20%   +0.01%     
==========================================
  Files         111      111              
  Lines       28777    28781       +4     
==========================================
+ Hits        26530    26537       +7     
+ Misses       2247     2244       -3     

Impacted Files	Coverage Δ
redis/connection.py	86.90% <100.00%> (ø)
tests/test_connection_pool.py	100.00% <100.00%> (ø)
redis/asyncio/connection.py	86.11% <0.00%> (+0.11%)	⬆️
tests/test_cluster.py	96.96% <0.00%> (+0.11%)	⬆️

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