$ python3.7 -m venv --without-pip venv
$ python3.7 -c "from urllib.request import urlretrieve; urlretrieve('https://bootstrap.pypa.io/get-pip.py', 'get-pip.py')"
$ venv/bin/python get-pip.py
Looking in indexes: ...
Looking in links: ...
Collecting pip
...
Collecting setuptools
...
Collecting wheel
...
Installing collected packages: wheel, setuptools, pip
Successfully installed pip-21.0.1 setuptools-53.0.0 wheel-0.36.2
$ . venv/bin/activate
(venv) $ cat > constraints
-c https://zopefoundation.github.io/Zope/releases/5.1/constraints.txt
Zope == 5.1
(venv) $ pip install pymongocrypt -c constraints
Looking in indexes: ... 
Looking in links: ...
Collecting pymongocrypt
...
ERROR: Cannot install pymongocrypt==1.0.0 and pymongocrypt==1.0.1 because these package versions have conflicting dependencies.

The conflict is caused by:
    pymongocrypt 1.0.1 depends on cryptography<3 and >=2.0
    pymongocrypt 1.0.0 depends on cryptography<3 and >=2.0

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies
(venv) $