Allow unsafe packages when compiling depdencies

pip-compile considers several packages "unsafe" for pinning. However, we
would like to be able to pin the pip version itself in requirements-dev
("pip" is one of these unsafe packages).

Adding "--allow-unsafe" tells pip-compile to pin versions of "unsafe"
packages. Despite the warning, there's no evidence of any unsafe effects
from this, and pip-tools is considering deprecating this flag and making
it the default behaviour anyway [1].

I've tested this locally and it seems to work without issue.

[1]: jazzband/pip-tools#806 (comment)

Makefile

@@ -4,5 +4,5 @@ sync:
 	pip-sync requirements.txt
 
 update:
-	pip-compile --quiet requirements.in --output-file requirements.txt
-	pip-compile --quiet requirements-dev.in --output-file requirements-dev.txt
+	pip-compile --allow-unsafe --quiet requirements.in --output-file requirements.txt
+	pip-compile --allow-unsafe --quiet requirements-dev.in --output-file requirements-dev.txt

requirements-dev.txt

@@ -2,7 +2,7 @@
 # This file is autogenerated by pip-compile
 # To update, run:
 #
-#    pip-compile --output-file=requirements-dev.txt requirements-dev.in
+#    pip-compile --allow-unsafe --output-file=requirements-dev.txt requirements-dev.in
 #
 appdirs==1.4.3            # via black
 astroid==2.4.0            # via pylint

requirements.txt

@@ -2,7 +2,7 @@
 # This file is autogenerated by pip-compile
 # To update, run:
 #
-#    pip-compile --output-file=requirements.txt requirements.in
+#    pip-compile --allow-unsafe --output-file=requirements.txt requirements.in
 #
 certifi==2019.9.11        # via requests
 chardet==3.0.4            # via requests