unittests: migrate from jsonschema to fastjsonschema

The former has rust dependencies, which lead to max capping on Cygwin
since there is no rust compiler there. But it turns out there are other
disadvantages of jsonschema:

- it involves installing 5 wheels, instead of just 1
- it is much slower

To give some perspective to the latter issue, this is what it looks like
when I test with jsonschema:

```
===== 1 passed, 509 deselected in 3.07s =====
Total time: 3.341 seconds
```

And here's what it looks like when I test with fastjsonschema:
```
===== 1 passed, 509 deselected, 1 warning in 0.28s =====
Total time: 0.550 seconds
```

I cannot think of a good reason to use the former. Although in order to
work on old CI images, we'll support it as a fallback mechanism

2 files changed, 2 insertions, 2 deletions

diff --git a/ci/ciimage/common.sh b/ci/ciimage/common.sh
index 67d5997..0169182 100644
--- a/ci/ciimage/common.sh
+++ b/ci/ciimage/common.sh
@@ -15,7 +15,7 @@ base_python_pkgs=(
   pytest-xdist
   pytest-subtests
   coverage
-  jsonschema
+  fastjsonschema
 )
 
 python_pkgs=(
diff --git a/ci/run.ps1 b/ci/run.ps1
index badb4a7..05bb6b6 100644
--- a/ci/run.ps1
+++ b/ci/run.ps1
@@ -92,7 +92,7 @@ python --version
 
 # Needed for running unit tests in parallel.
 echo ""
-python -m pip --disable-pip-version-check install --upgrade pefile pytest-xdist pytest-subtests jsonschema coverage
+python -m pip --disable-pip-version-check install --upgrade pefile pytest-xdist pytest-subtests fastjsonschema coverage
 
 # Needed for running the Cython tests
 python -m pip --disable-pip-version-check install cython