Update dependency isort to v5

See merge request !19

Update dependency pytest to v6

See merge request !21

Update dependency flake8-isort to v4

See merge request !22

Resolve "Enforce extra ldap on core dependency"

Closes #20

See merge request !18

Support LDAPSearchUnion now used in core

See merge request !13

Update dependency flake8-black to ^0.2.0

See merge request !15

Testsuite/Lint

See merge request !14

We expect it to be equal or all searches. If not, there is really
nothing that can be done for that LDAP admin.

Related to AlekSIS#249

Merge branch '17-no-such-preference-in-sitepreferenceregistry-with-section-ldap-and-name' into 'master'

Resolve "No such preference in SitePreferenceRegistry with section=ldap and name=additional_field_core_person_street_request"

Closes #17

See merge request !11

Resolve "Broken format strings in preferences"

Closes #18

See merge request !12

Currently translated at 100.0% (22 of 22 strings)

Translation: AlekSIS/AlekSIS-App-LDAP
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-ldap/de/

This reverts commit 1132fc11.

# Conflicts:
#	aleksis/apps/ldap/util/ldap_sync.py