Use only-if-needed upgrade strategy to install deps for PRs

[kaxil]

Currently, upgrading dependencies in setup.py still run with previous versions of the package for the PR which fails.

This PR will change it to only upgrade the package if a requirement is not satisfied (i.e. if we change the upper or lower bound of dependency in setup.py).

This will avoid the situation like #11020 and #11313 where we can't test upgrading the package in the PR itself. And we were "deadlocked" even after merging the PR as the constraints won't allow installing the new version of the package, I had to generate new constraints myself and push to constraints-master branch (e6e840f), this is not ideal and won't be possible for just contributors.

Using only-if-needed upgrade strategy - packages are only upgraded if they are named in the pip command or a requirement file (i.e, they are direct requirements), or an upgraded parent needs a later version of the dependency than is currently installed.

Example:

Using only-if-needed:

root@822984cd3cf4:/opt/airflow# pip install -e . --upgrade --upgrade-strategy only-if-needed
Obtaining file:///opt/airflow
Requirement already satisfied, skipping upgrade: alembic<2.0,>=1.2 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (1.4.3)
Requirement already satisfied, skipping upgrade: jinja2<2.12.0,>=2.10.1 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (2.11.2)
Requirement already satisfied, skipping upgrade: json-merge-patch==0.2 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (0.2)
Requirement already satisfied, skipping upgrade: tabulate<0.9,>=0.7.5 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (0.8.7)
Requirement already satisfied, skipping upgrade: graphviz>=0.12 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (0.14.1)
Collecting tenacity~=6.2.0
  Downloading tenacity-6.2.0-py2.py3-none-any.whl (24 kB)
...
...

vs

Using eager:

root@822984cd3cf4:/opt/airflow# pip install -e . --upgrade --upgrade-strategy eager
Obtaining file:///opt/airflow
Requirement already up-to-date: alembic<2.0,>=1.2 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (1.4.3)
Requirement already up-to-date: argcomplete~=1.10 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (1.12.1)
Requirement already up-to-date: cryptography>=0.9.3 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (3.1.1)
Requirement already up-to-date: dill<0.4,>=0.2.2 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (0.3.2)
Requirement already up-to-date: funcsigs<2.0.0,>=1.0.0 in /usr/local/lib/python3.7/site-packages (from apache-airflow==2.0.0.dev0) (1.0.2)
Collecting graphviz>=0.12
  Downloading graphviz-0.14.2-py2.py3-none-any.whl (18 kB)
....
....

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[kaxil]

#11020 is blocked without this PR

cc @baolsen

[potiuk]

Cool!