- Fix documentation build

- Add textproc/py-sphinx_rtd_theme and textproc/towncrier to build dependencies
- Add files/patch-docs_conf.py to fix ModuleNotFoundError (virtualenv)
- Set LANG and LC_ALL environment variables to mitigate a RuntimeError during the
  build stage of the doc.:

  RuntimeError: Click will abort further execution because Python 3 was configured to use ASCII as encoding for the environment. Consult https://click.palletsprojects.com/en/7.x/python3/ for mitigation steps.

devel/py-virtualenv/Makefile

@@ -14,9 +14,16 @@ LICENSE=	MIT
 TEST_DEPENDS=	${PYTHON_PKGNAMEPREFIX}pytest>0:devel/py-pytest@${PY_FLAVOR} \
 		${PYTHON_PKGNAMEPREFIX}mock>0:devel/py-mock@${PY_FLAVOR}
 
-DOCS_BUILD_DEPENDS=	${PYTHON_PKGNAMEPREFIX}sphinx>=1.1.3:textproc/py-sphinx@${PY_FLAVOR}
+DOCS_BUILD_DEPENDS=	${PYTHON_PKGNAMEPREFIX}sphinx>=1.1.3:textproc/py-sphinx@${PY_FLAVOR} \
+			${PYTHON_PKGNAMEPREFIX}sphinx_rtd_theme>0:textproc/py-sphinx_rtd_theme@${PY_FLAVOR} \
+			${PYTHON_PKGNAMEPREFIX}towncrier>0:textproc/py-towncrier@${PY_FLAVOR}
 DOCS_PORTDOCS=	*
-DOCS_VARS=	PYDISTUTILS_BUILD_TARGET+=build_sphinx
+DOCS_VARS=	PYDISTUTILS_BUILD_TARGET="build build_sphinx"
+
+# Required by towncrier/click when Python < 3.7
+# see https://click.palletsprojects.com/en/7.x/python3/
+# Documentation is in English, so...
+MAKE_ENV=	LANG=en_US.UTF-8 LC_ALL=en_US.UTF-8
 
 GH_ACCOUNT=	pypa
 
@@ -24,6 +31,8 @@ NO_ARCH=	yes
 
 OPTIONS_DEFINE=	DOCS
 
+PYDISTUTILS_BUILDARGS=	-c ${WRKSRC}/docs
+
 USES=		python
 USE_GITHUB=	yes
 USE_PYTHON=	autoplist concurrent distutils

devel/py-virtualenv/files/patch-docs_conf.py

@@ -0,0 +1,10 @@
+--- docs/conf.py.orig	2019-09-03 16:32:54 UTC
++++ docs/conf.py
+@@ -5,6 +5,7 @@ import re
+ import subprocess
+ import sys
+ from pathlib import Path
++sys.path.insert(0, os.path.abspath('../build/lib'))
+ 
+ from virtualenv import __version__
+