fix tests: changed random bucket name to full lowercase

[simon-saliba]

This PR updates the random bucket naming generation used in tests to match the new AWS S3 bucket naming specifications.

According to documentation [https://docs.aws.amazon.com/AmazonS3/latest/userguide/bucketnamingrules.html],
bucket names can consist only of lowercase letters, numbers, dots (.), and hyphens (-).

S3 bucket url skeletons have also changed to take the following form:
https://bucket-name.s3.Region.amazonaws.com/key-name
according to source [https://docs.aws.amazon.com/AmazonS3/latest/userguide/access-bucket-intro.html]

These couple of changes have been making a number of unit tests in test_basic_s3.py failing with error: InvalidBucketName, and NoSuckBucket.

Description of Change

Change required to make all unit tests under test_basic_s3.py pass.
Changes include:

• making random S3 bucket names used in tests only lowercase
• changing the url for S3 buckets

Assumptions

Replace this text with any assumptions made (if any)

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

Merging #919 (c46e518) into master (0b71254) will decrease coverage by 1.56%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #919      +/-   ##
==========================================
- Coverage   89.27%   87.70%   -1.57%     
==========================================
  Files          42       45       +3     
  Lines        4576     4848     +272     
==========================================
+ Hits         4085     4252     +167     
- Misses        491      596     +105     

Flag	Coverage Δ
unittests	87.70% <100.00%> (-1.57%)	⬇️

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

Impacted Files	Coverage Δ
tests/conftest.py	91.72% <ø> (+0.40%)	⬆️
tests/test_basic_s3.py	85.99% <100.00%> (+0.70%)	⬆️
aiobotocore/retries/adaptive.py	83.72% <0.00%> (ø)
aiobotocore/retries/bucket.py	66.66% <0.00%> (ø)
tests/test_adaptive.py	36.13% <0.00%> (ø)
tests/test_patches.py	96.07% <0.00%> (+0.16%)	⬆️
aiobotocore/client.py	84.90% <0.00%> (+1.23%)	⬆️

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[thehesiod]

@simon-saliba thank you!