[doc] improve blog tutorial first steps and explain common errors

[doc] new behavior in virtualenv since 3.26

[skeleton] use python3 flake8 In tox "flake8" environment, use python 3 virtualenv, so we can check for python3 incompatible syntax.

[skeleton] drop whitelist_externals for flake8 whitelist_externals makes sense when used with sitepackages=true. In this case it will always rely on $VIRTUAL_ENV/bin/flake8 so, the whitelist_externals has no effect.

Fix flake8 issues since release 3.6.0 Flake8 had a new release which raise new issues, namely: W504: line break after binary operator F841: local variable 'ex' is assigned to but never used W605: invalid escape sequence F821: undefined name 'buffer' (noqa seems the only way to avoid this false positive) Also pin flake8>=3.6 in our tests and make explicit that we use python3 to run flake8 tests.

Fix typo in RDFS URI

[server] Fixed issue with the adapters cache of a cloned EditedEntity The adapters' cache of the entity within an EditedEntity used to be copied along the entity itself during a cloning operation of the EditedEntity. This raises an issue when the adapters' cache is not empty and contains cached values linking to transaction-specific data (or the transaction itself). This prevents the clone EditedEntity being used in a transaction outside the one that created it. This change resets the adapters' cache of the new copy so that no cached value remain. The new copy can then be used in a new transaction afterwards.

[py37] `async` is now a reserved keyword

[skeleton] Use "py3" as tox environment instead of "py34"

[views/startup] drop a duplicate rql query in manage view Counting entities in `nb` is already done above this code, regression introduced by a85db6fa

Depend on cubicweb-file >= 2.0.0 I applied V1 of the patch, but V2 contained this extra hunk...

[test] Update webconfig's tests accounting for cubicweb-file being now a package WebconfigTC.test_locate_all_files and WebconfigTC.test_locate_resource assumed that cubicweb-file was installed in <share>/cubes/file whereas, from the latest release, it is now a standard Python package thus installed in site-packages as cubicweb_file. We simply update base directory of these two tests.

[widgets.js] fix obvious bug detected by sonar

[views/uicfg] obvious bug detected by sonar

[style] Make toolsutils.py flake8-clean Run `autopep8 --max-line-length 100 -i cubicweb/toolsutils.py` and fixed unused imports.

[test/rset] typo