Pin build process to Python 3.7

After a package update, transcrypt produces incomplete JS:
__target__/OOMAnalyser.js (147:15)
145:       }
146:       if (cols_to_strip) {
147:         var line = ;
                        ^
148:       }
149:       if (line.startswith ('kernel:')) {

instead of
 146       if (cols_to_strip) {
 147         var line = __getitem__ ((function () {
 148           var __accu0__ = line;
 149           return __call__ (__accu0__.py_split, __accu0__, '     ', cols_to_strip);
 150         }) (), __neg__ (1));
 151       }
 152       if (line.startswith ('kernel:')) {

A temporary pinning of Transcrypt to Python 3.7 solves this issue.

.build.yml

@@ -33,6 +33,16 @@ tasks:
       git clone https://aur.archlinux.org/rollup.git
       cd rollup
       makepkg --noconfirm -si
+  - setup_py37: |
+      cd OOMAnalyser
+      git clone https://aur.archlinux.org/python37.git
+      cd python37
+      makepkg --noconfirm -si --skippgpcheck
+      cd ..
+      # create virtualenv for Python 3.7
+      make venv-clean
+      virtualenv3 -p /usr/bin/python3.7 env
+      env/bin/pip  install -Ur requirements.txt
   - build: |
       cd OOMAnalyser
       make build

Makefile

@@ -10,7 +10,7 @@
 SHELL             = /bin/sh
 
 BASE_DIR          = .
-PYTHON3_BIN       = python3
+PYTHON3_BIN       = /usr/bin/python3.7
 ROLLUP_BIN        = rollup
 ROLLUP_OPTS       = --format=umd --name OOMAnalyser --file=OOMAnalyser.js
 TRANSCRYPT_BIN    = transcrypt
@@ -47,7 +47,7 @@ distclean: clean venv-clean
 	@(RM) --force .git .gitignore
 
 $(VIRTUAL_ENV_DIR)/bin/activate: requirements.txt
-	test -d $(VIRTUAL_ENV_DIR) || virtualenv $(VIRTUAL_ENV_DIR)
+	test -d $(VIRTUAL_ENV_DIR) || virtualenv -p $(PYTHON3_BIN) $(VIRTUAL_ENV_DIR)
 	. $(VIRTUAL_ENV_DIR)/bin/activate
 	$(VIRTUAL_ENV_DIR)/bin/pip install -Ur requirements.txt
 	touch $(VIRTUAL_ENV_DIR)/bin/activate

README.md

@@ -34,7 +34,7 @@ without an Internet connection.
 
 ### Requirements
 
- * [Python](http://www.python.org) 3.6 or later
+ * [Python](http://www.python.org) 3.7
  * [Transcrypt](https://www.transcrypt.org/) 3.7
  * [Rollup](https://rollupjs.org/)