Revert Pygments backward incompatibility work-around

The issue was fixed in upstream's 8.2.2 release.  See
getnikola/nikola#3617 (comment).

This commit reverts 1af4168.

.dockerignore

@@ -1,3 +1,2 @@
 *
 !Dockerfile
-!constraints.txt

Dockerfile

@@ -9,7 +9,6 @@ MAINTAINER  Olaf Meeuwissen <[email protected]>
 ENV PIP_OPTS --no-cache-dir --disable-pip-version-check
 ARG _VERSION
 
-COPY constraints.txt /tmp/constraints.txt
 RUN apk add --no-cache                                                  \
         python3                                                         \
         py3-pip                                                         \
@@ -27,8 +26,7 @@ RUN apk add --no-cache                                                  \
         jpeg-dev                                                        \
                                                                      && \
     CFLAGS="$CFLAGS -L/lib"                                             \
-    pip3 install -c /tmp/constraints.txt $PIP_OPTS                      \
-         nikola$_VERSION                                                \
+    pip3 install $PIP_OPTS nikola$_VERSION                              \
                                                                      && \
     apk del .build-deps                                              && \
     find /usr/lib/python3.*                                             \
@@ -47,8 +45,7 @@ RUN apk add --no-cache                                                  \
         python3-dev                                                     \
         zeromq-dev                                                      \
                                                                      && \
-    pip3 install -c /tmp/constraints.txt $PIP_OPTS                      \
-         'nikola[extras]'$_VERSION                                      \
+    pip3 install $PIP_OPTS 'nikola[extras]'$_VERSION                    \
                                                                      && \
     apk del .extra-build-deps                                        && \
     find /usr/lib/python3.*                                             \