Fix building on FreeBSD

Cirrus CI on FreeBSD failed because "No packages available to install
matching 'py37-pip' have been found in the repositories"i [1].
Package 'py37-pip' has been replaced with 'py38-pip', see [2].

1. https://cirrus-ci.com/task/6375178809638912
2. https://www.freshports.org/devel/py-pip/

.cirrus.yml

@@ -64,10 +64,10 @@ freebsd_12_2: &freebsd_12_2
   env:
     CC: clang
   install_script:
-    - ASSUME_ALWAYS_YES=yes pkg bootstrap -f; pkg install -y cmake fusefs-libs pkgconf python3 py37-pip fio
+    - ASSUME_ALWAYS_YES=yes pkg bootstrap -f; pkg install -y cmake fusefs-libs pkgconf python3 py38-pip fio
     - kldload fuse
     - kldstat
-    - sudo python3.7 -m pip install pytest
+    - sudo python3.8 -m pip install pytest
 
 task:
   matrix: